Metathesis polymerization methods

ABSTRACT

The present disclosure is directed to methods of making a polymer, including exposing a reaction mixture including a strained cyclic unsaturated monomer and an organic initiator to a stimulus to provide an activated organic initiator, whereby the activated organic initiator is effective to polymerize the strained cyclic unsaturated monomer via a 4-membered carbocyclic intermediate to provide a polymer having constitutional units derived from the strained cyclic unsaturated monomer.

CROSS-REFERENCE(S) TO RELATED APPLICATION(S)

This application is a Continuation of U.S. patent application Ser. No.15/506,578, filed on Feb. 24, 2017, which is a National Stage ofPCT/US2015/048395, filed on Sep. 3, 2015, which claims the benefit ofU.S. Patent Application No. 62/045,271, filed Sep. 3, 2014; U.S. PatentApplication No. 62/101,263, filed Jan. 8, 2015; U.S. Patent ApplicationNo. 62/136,069, filed Mar. 20, 2015; and U.S. Patent Application No.62/171,735, filed Jun. 5, 2015; the disclosures of which are herebyincorporated by reference in their entirety.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Grant no.W911NF-15-1-0139 awarded by the Army Research Office. The government hascertain rights in the invention.

BACKGROUND

Ring-opening metathesis polymerization (ROMP) is a popular method forthe preparation of a variety of functional polymers and is one of themost prevalent technologies that has emerged from the development oftransition metal-based olefin metathesis catalysts.

Applications in areas such as drug delivery, biomedical engineering,photovoltaics, and production of structural materials have eachbenefited from developments in ROMP methods. In general, ROMP is used toachieve living polymerizations, to provide polymers of narrowdispersity, to enable control over end group functionality, and toincorporate a broad range of functional groups into polymer scaffoldsand network materials.

Traditional ROMP initiators include transition metal complexes, such asRu, W, or Mo-alkylidene complexes, along with a number of ill-definedspecies containing various mixtures of metal salts. Examples of Ru-,Mo-, and W-based alkylidene initiators are shown in Scheme 1 below.

It is believed that each of these initiators proceeds through the samegeneral mechanism involving a metallacyclobutane intermediate as shownin Scheme 2, below, where M is a metal and R is a substituent.

Despite the positive attributes that traditional transition metalcatalysts can provide to a ROMP process, a significant disadvantagecommon to transition metal-catalyzed ROMP is that metal-based byproductscan be difficult to remove from the polymeric materials. This can leadto complications with biological studies, conductivity measurements, oroptical properties. Moreover, downstream reactivity of residual metallicspecies can also be problematic. At a minimum, the potential for metalcontaminants often warrants quantitation by advanced techniques, such asinductively-coupled plasma mass spectrometry. Indeed, these issues havemotivated a number of protocols for removing metal-based components,which, even when successful, add additional processing steps formaterial production.

Thus, there is presently a need for polymerization using organicinitiators, for cross-linking reactions that do not require metalcatalysts, and for reaction products that do not have trapped metals.The present disclosure seeks to fulfill these needs and provides furtherrelated advantages.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In one aspect, this disclosure features a method of making a polymerincluding exposing a reaction mixture including a strained cyclicunsaturated monomer and an organic initiator to a stimulus to provide anactivated organic initiator, whereby the activated organic initiator iseffective to polymerize the strained cyclic unsaturated monomer, toprovide a polymer having constitutional units derived from the strainedcyclic unsaturated monomer.

In another aspect, this disclosure features a method of making a polymerincluding exposing a reaction mixture including a strained cyclicunsaturated monomer, an organic unsaturated initiator, and aco-initiator, to a stimulus to provide an activated co-initiator whichactivates the organic unsaturated initiator, whereby the activatedorganic unsaturated initiator is effective to polymerize the strainedcyclic unsaturated monomer, to provide a polymer having constitutionalunits derived from the strained cyclic unsaturated monomer.

In yet another aspect, this disclosure features a polymer, including analkenyl substituted with a C₁-C₂₀ alkoxy moiety at a polymer terminusand wherein the polymer is metal-free.

In yet another aspect, this disclosure features an article ofmanufacture, including the polymer produced by any of the methods above.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings.

FIGS. 1A and 1B are graphs showing M_(n) (circles) and Ð (triangles) vs% conversion of an embodiment of a monomer using initial monomer toinitiator 1:2c ratio of 100:1 (FIG. 1A) and 500:1 (FIG. 1B).

FIG. 2 is graph showing % conversion of an embodiment of a monomer vstime, solid lines indicate periods of exposure to blue LED light. Dottedlines indicate periods in the dark, data point labels indicate Mn values(kDa). Initial conditions:monomer to initiator 1:2a ratio=100:1,[1]₀=1.9 M.

FIG. 3 is a graph showing Mn (⋅) and % dicyclopentadiene (“DCPD”)incorporated into an embodiment of a polymer polymer (Δ) vs. % DCPDloaded.

FIG. 4 is an illustration of embodiments of reaction mechanisms andmonomers.

FIG. 5 is a graph showing the conversion vs. time for embodiments ofmonomers (monomers 2 (unfilled circle), 6 (filled circles), 7 (unfilledtriangles), and 8 (filled triangles)) as determined by 1H-NMRspectroscopy.

FIG. 6 is a schematic illustration of an embodiment of a crosslinkingreaction of polyDCPD using a thiol-ene reaction (top) and a photograph(bottom) of an embodiment of a polymer in THF before crosslinking(bottom left) and after UV promoted crosslinking (bottom right).

DETAILED DESCRIPTION

The present disclosure is directed to methods of making a polymer,including exposing a reaction mixture including a strained cyclicunsaturated monomer and an organic initiator to a stimulus to provide anactivated organic initiator, whereby the activated organic initiator iseffective to polymerize the strained cyclic unsaturated monomer via a4-membered carbocyclic intermediate to provide a polymer havingconstitutional units derived from the strained cyclic unsaturatedmonomer. The 4-membered carbocyclic intermediate can be formed by a[2+2] cycloaddition of the activated organic initiator and the strainedcyclic unsaturated monomer.

The ROMP process can occur in a metal-free manner, using organicinitiators that are metal-free. Thus, the resulting polymers can bemetal-free. Without wishing to be bound by theory, it is believed thatthe polymerization process outcompetes reductive quenching reactionsthat may occur during the reaction, which otherwise afford cyclobutanesin a single olefin cross-methathesis reaction rather than a desiredpolymer by a polymerization propagation reaction.

The ROMP process and resulting polymers of the present disclosure havenumerous advantages, such as obviating the need for removing metal-basedcomponents from polymers, providing polymerization processes that offerunique control over polymer end group functionality, providing polymershaving certain main chain microstructures, and providing methods forspatiotemporal control over polymer production.

Definitions

At various places in the present specification, substituents ofcompounds of the disclosure are disclosed in groups or in ranges. It isspecifically intended that the disclosure include each and everyindividual subcombination of the members of such groups and ranges. Forexample, the term “C₁₋₆ alkyl” is specifically intended to individuallydisclose methyl, ethyl, C₃ alkyl, C₄ alkyl, C₅ alkyl, and C₆ alkyl.

It is further appreciated that certain features of the disclosure, whichare, for clarity, described in the context of separate embodiments, canalso be provided in combination in a single embodiment.

Conversely, various features of the disclosure which are, for brevity,described in the context of a single embodiment, can also be providedseparately or in any suitable subcombination.

Groupings of alternative elements or embodiments of the inventiondisclosed herein are not to be construed as limitations. Each groupmember may be referred to and claimed individually or in any combinationwith other members of the group or other elements found herein. It isanticipated that one or more members of a group may be included in, ordeleted from, a group for reasons of convenience and/or patentability.When any such inclusion or deletion occurs, the specification is deemedto contain the group as modified thus fulfilling the written descriptionof all Markush groups used in the appended claims.

The terms “a,” “an,” “the” and similar referents used in the context ofdescribing the invention (especially in the context of the followingclaims) are to be construed to cover both the singular and the plural,unless otherwise indicated herein or clearly contradicted by context.Recitation of ranges of values herein is merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range. Unless otherwise indicated herein, eachindividual value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention otherwise claimed. No languagein the specification should be construed as indicating any non-claimedelement essential to the practice of the invention.

As will be understood by one of ordinary skill in the art, eachembodiment disclosed herein can comprise, consist essentially of orconsist of its particular stated element, step, ingredient or component.As used herein, the transition term “comprise” or “comprises” meansincludes, but is not limited to, and allows for the inclusion ofunspecified elements, steps, ingredients, or components, even in majoramounts. The transitional phrase “consisting of” excludes any element,step, ingredient or component not specified. The transition phrase“consisting essentially of” limits the scope of the embodiment to thespecified elements, steps, ingredients or components and to those thatdo not materially affect the embodiment.

Furthermore, references have been made to patents and printedpublications throughout this specification. Each of the above-citedreferences and printed publications are individually incorporated hereinby reference in their entirety.

As used herein, the term “substituted” or “substitution” refers to thereplacing of a hydrogen atom with a substituent other than H. Forexample, an “N-substituted piperidin-4-yl” refers to replacement of theH atom from the NH of the piperidinyl with a non-hydrogen substituentsuch as, for example, alkyl.

Terms used herein may be preceded and/or followed by a single dash, “—”,or a double dash, “═”, to indicate the bond order of the bond betweenthe named substituent and its parent moiety; a single dash indicates asingle bond and a double dash indicates a double bond. In the absence ofa single or double dash it is understood that a single bond is formedbetween the substituent and its parent moiety; further, substituents areintended to be read “left to right” unless a dash indicates otherwise.For example, C₁-C₆alkoxycarbonyloxy and —OC(O)C₁-C₆alkyl indicate thesame functionality; similarly arylalkyl and -alkylaryl indicate the samefunctionality.

As used herein, the term “alkyl” refers to a straight or branched chainhydrocarbon containing from 1 to 10 carbon atoms, unless otherwisespecified. Representative examples of alkyl include, but are not limitedto, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl,tert-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl, 3-methylhexyl,2,2-dimethylpentyl, 2,3-dimethylpentyl, n-heptyl, n-octyl, n-nonyl, andn-decyl.

As used herein, the term “alkylene” refers to a linking alkyl group. Thelinking alkyl group can be a straight or branched chain; examplesinclude, but are not limited to —CH₂—, —CH₂CH₂—, —CH₂CH₂CHC(CH₃)—, and—CH₂CH(CH₂CH₃)CH₂—.

As used herein, the term “alkenyl” refers to a straight or branchedchain hydrocarbon containing from 2 to 10 carbons, unless otherwisespecified, and containing at least one carbon-carbon double bond.Representative examples of alkenyl include, but are not limited to,ethenyl, 2-propenyl, 2-methyl-2-propenyl, 3-butenyl, 4-pentenyl,5-hexenyl, 2-heptenyl, 2-methyl-1-heptenyl, 3-decenyl, and3,7-dimethylocta-2,6-dienyl.

As used herein, the term “alkenylene” refers to a linking alkenyl group.

As used herein, the term “alkynyl” refers to a straight or branchedchain hydrocarbon group containing from 2 to 10 carbon atoms andcontaining at least one carbon-carbon triple bond. Representativeexamples of alkynyl include, but are not limited, to acetylenyl,1-propynyl, 2-propynyl, 3-butynyl, 2-pentynyl, and 1-butynyl.

As used herein, “alkynylene” refers to a linking alkynyl group.

As used herein, the term “aryl” refers to a phenyl (i.e., monocyclicaryl), a bicyclic ring system containing at least one phenyl ring or anaromatic bicyclic ring containing only carbon atoms in the aromaticbicyclic ring system or a multicyclic aryl ring system, provided thatthe bicyclic or multicyclic aryl ring system does not contain aheteroaryl ring when fully aromatic. The bicyclic aryl can be azulenyl,naphthyl, or a phenyl fused to a monocyclic cycloalkyl, a monocycliccycloalkenyl, or a monocyclic heterocyclyl. The bicyclic aryl isattached to the parent molecular moiety through any carbon atomcontained within the phenyl portion of the bicyclic system, or anycarbon atom with the naphthyl or azulenyl ring. The fused monocycliccycloalkyl or monocyclic heterocyclyl portions of the bicyclic aryl areoptionally substituted with one or two oxo and/or thia groups.Representative examples of the bicyclic aryls include, but are notlimited to, azulenyl, naphthyl, dihydroinden-1-yl, dihydroinden-2-yl,dihydroinden-3-yl, dihydroinden-4-yl, 2,3-dihydroindol-4-yl,2,3-dihydroindol-5-yl, 2,3-dihydroindol-6-yl, 2,3-dihydroindol-7-yl,inden-1-yl, inden-2-yl, inden-3-yl, inden-4-yl, dihydronaphthalen-2-yl,dihydronaphthalen-3-yl, dihydronaphthalen-4-yl, dihydronaphthalen-1-yl,5,6,7,8-tetrahydronaphthalen-1-yl, 5,6,7,8-tetrahydronaphthalen-2-yl,2,3-dihydrobenzofuran-4-yl, 2,3-dihydrobenzofuran-5-yl,2,3-dihydrobenzofuran-6-yl, 2,3-dihydrobenzofuran-7-yl,benzo[d][1,3]dioxol-4-yl, benzo[d][1,3]dioxol-5-yl,2H-chromen-2-on-5-yl, 2H-chromen-2-on-6-yl, 2H-chromen-2-on-7-yl,2H-chromen-2-on-8-yl, isoindoline-1,3-dion-4-yl,isoindoline-1,3-dion-5-yl, inden-1-on-4-yl, inden-1-on-5-yl,inden-1-on-6-yl, inden-1-on-7-yl, 2,3-dihydrobenzo[b][1,4]dioxan-5-yl,2,3-dihydrobenzo[b][1,4]dioxan-6-yl,2H-benzo[b][1,4]oxazin3(4H)-on-5-yl, 2H-benzo[b][1,4]oxazin3(4H)-on-6-yl, 2H-benzo[b][1,4]oxazin3(4H)-on-7-yl,2H-benzo[b][1,4]oxazin3(4H)-on-8-yl, benzo[d]oxazin-2(3H)-on-5-yl,benzo[d]oxazin-2(3H)-on-6-yl, benzo[d]oxazin-2(3H)-on-7-yl,benzo[d]oxazin-2(3H)-on-8-yl, quinazolin-4(3H)-on-5-yl,quinazolin-4(3H)-on-6-yl, quinazolin-4(3H)-on-7-yl,quinazolin-4(3H)-on-8-yl, quinoxalin-2(1H)-on-5-yl,quinoxalin-2(1H)-on-6-yl, quinoxalin-2(1H)-on-7-yl,quinoxalin-2(1H)-on-8-yl, benzo[d]thiazol-2(3H)-on-4-yl,benzo[d]thiazol-2(3H)-on-5-yl, benzo[d]thiazol-2(3H)-on-6-yl, and,benzo[d]thiazol-2(3H)-on-7-yl. In certain embodiments, the bicyclic arylis (i) naphthyl or (ii) a phenyl ring fused to either a 5 or 6 memberedmonocyclic cycloalkyl, a 5 or 6 membered monocyclic cycloalkenyl, or a 5or 6 membered monocyclic heterocyclyl, wherein the fused cycloalkyl,cycloalkenyl, and heterocyclyl groups are optionally substituted withone or two groups which are independently oxo or thia. Multicyclic arylgroups are a phenyl ring (base ring) fused to either (i) one ring systemselected from the group consisting of a bicyclic aryl, a bicycliccycloalkyl, a bicyclic cycloalkenyl, and a bicyclic heterocyclyl; or(ii) two other ring systems independently selected from the groupconsisting of a phenyl, a bicyclic aryl, a monocyclic or bicycliccycloalkyl, a monocyclic or bicyclic cycloalkenyl, and a monocyclic orbicyclic heterocyclyl, provided that when the base ring is fused to abicyclic cycloalkyl, bicyclic cycloalkenyl, or bicyclic heterocyclyl,then the base ring is fused to the base ring of the a bicycliccycloalkyl, bicyclic cycloalkenyl, or bicyclic heterocyclyl. Themulticyclic aryl is attached to the parent molecular moiety through anycarbon atom contained within the base ring. In certain embodiments,multicyclic aryl groups are a phenyl ring (base ring) fused to either(i) one ring system selected from the group consisting of a bicyclicaryl, a bicyclic cycloalkyl, a bicyclic cycloalkenyl, and a bicyclicheterocyclyl; or (ii) two other ring systems independently selected fromthe group consisting of a phenyl, a monocyclic cycloalkyl, a monocycliccycloalkenyl, and a monocyclic heterocyclyl, provided that when the basering is fused to a bicyclic cycloalkyl, bicyclic cycloalkenyl, orbicyclic heterocyclyl, then the base ring is fused to the base ring ofthe a bicyclic cycloalkyl, bicyclic cycloalkenyl, or bicyclicheterocyclyl. Examples of multicyclic aryl groups include but are notlimited to anthracen-9-yl and phenanthren-9-yl.

As used herein, the term “arylene” refers to a linking aryl group.

As used herein, the term “cycloalkyl” refers to a monocyclic, bicyclic,or a multicyclic cycloalkyl ring system. Monocyclic ring systems arecyclic hydrocarbon groups containing from 3 to 8 carbon atoms, wheresuch groups can be saturated or unsaturated, but not aromatic. Incertain embodiments, cycloalkyl groups are fully saturated. Examples ofmonocyclic cycloalkyls include cyclopropyl, cyclobutyl, cyclopentyl,cyclopentenyl, cyclohexyl, cyclohexenyl, cycloheptyl, and cyclooctyl.Bicyclic cycloalkyl ring systems are bridged monocyclic rings or fusedbicyclic rings. Bridged monocyclic rings contain a monocyclic cycloalkylring where two non-adjacent carbon atoms of the monocyclic ring arelinked by an alkylene bridge of between one and three additional carbonatoms (i.e., a bridging group of the form —(CH₂)_(w)—, where w is 1, 2,or 3). Representative examples of bicyclic ring systems include, but arenot limited to, bicyclo[3.1.1]heptane, bicyclo[2.2.1]heptane,bicyclo[2.2.2]octane, bicyclo[3.2.2]nonane, bicyclo[3.3.1]nonane, andbicyclo[4.2.1]nonane. Fused bicyclic cycloalkyl ring systems contain amonocyclic cycloalkyl ring fused to either a phenyl, a monocycliccycloalkyl, a monocyclic cycloalkenyl, a monocyclic heterocyclyl, or amonocyclic heteroaryl. The bridged or fused bicyclic cycloalkyl isattached to the parent molecular moiety through any carbon atomcontained within the monocyclic cycloalkyl ring. Cycloalkyl groups areoptionally substituted with one or two groups which are independentlyoxo or thia. In certain embodiments, the fused bicyclic cycloalkyl is a5 or 6 membered monocyclic cycloalkyl ring fused to either a phenylring, a 5 or 6 membered monocyclic cycloalkyl, a 5 or 6 memberedmonocyclic cycloalkenyl, a 5 or 6 membered monocyclic heterocyclyl, or a5 or 6 membered monocyclic heteroaryl, wherein the fused bicycliccycloalkyl is optionally substituted by one or two groups which areindependently oxo or thia. Multicyclic cycloalkyl ring systems are amonocyclic cycloalkyl ring (base ring) fused to either (i) one ringsystem selected from the group consisting of a bicyclic aryl, a bicyclicheteroaryl, a bicyclic cycloalkyl, a bicyclic cycloalkenyl, and abicyclic heterocyclyl; or (ii) two other rings systems independentlyselected from the group consisting of a phenyl, a bicyclic aryl, amonocyclic or bicyclic heteroaryl, a monocyclic or bicyclic cycloalkyl,a monocyclic or bicyclic cycloalkenyl, and a monocyclic or bicyclicheterocyclyl. The multicyclic cycloalkyl is attached to the parentmolecular moiety through any carbon atom contained within the base ring.In certain embodiments, multicyclic cycloalkyl ring systems are amonocyclic cycloalkyl ring (base ring) fused to either (i) one ringsystem selected from the group consisting of a bicyclic aryl, a bicyclicheteroaryl, a bicyclic cycloalkyl, a bicyclic cycloalkenyl, and abicyclic heterocyclyl; or (ii) two other rings systems independentlyselected from the group consisting of a phenyl, a monocyclic heteroaryl,a monocyclic cycloalkyl, a monocyclic cycloalkenyl, and a monocyclicheterocyclyl. Examples of multicyclic cycloalkyl groups include, but arenot limited to tetradecahydrophenanthrenyl, perhydrophenothiazin-1-yl,and perhydrophenoxazin-1-yl.

As used herein, “cycloalkylene” refers to a linking cycloalkyl group.

As used herein, “cycloalkenyl” refers to a monocyclic, bicyclic, or amulticyclic cycloalkenyl ring system. Monocyclic ring systems are cyclichydrocarbon groups containing from 3 to 8 carbon atoms, where suchgroups are unsaturated (i.e., containing at least one annularcarbon-carbon double bond), but not aromatic. Examples of monocyclicring systems include cyclopentenyl and cyclohexenyl. Bicycliccycloalkenyl rings are bridged monocyclic rings or fused bicyclic rings.Bridged monocyclic rings contain a monocyclic cycloalkenyl ring wheretwo non-adjacent carbon atoms of the monocyclic ring are linked by analkylene bridge of between one and three additional carbon atoms (i.e.,a bridging group of the form —(CH₂)_(w)—, where w is 1, 2, or 3).Representative examples of bicyclic cycloalkenyls include, but are notlimited to, norbornenyl and bicyclo[2.2.2]oct-2-enyl. Fused bicycliccycloalkenyl ring systems contain a monocyclic cycloalkenyl ring fusedto either a phenyl, a monocyclic cycloalkyl, a monocyclic cycloalkenyl,a monocyclic heterocyclyl, or a monocyclic heteroaryl. The bridged orfused bicyclic cycloalkenyl is attached to the parent molecular moietythrough any carbon atom contained within the monocyclic cycloalkenylring. Cycloalkenyl groups are optionally substituted with one or twogroups which are independently oxo or thia. Multicyclic cycloalkenylrings contain a monocyclic cycloalkenyl ring (base ring) fused to either(i) one ring system selected from the group consisting of a bicyclicaryl, a bicyclic heteroaryl, a bicyclic cycloalkyl, a bicycliccycloalkenyl, and a bicyclic heterocyclyl; or (ii) two rings systemsindependently selected from the group consisting of a phenyl, a bicyclicaryl, a monocyclic or bicyclic heteroaryl, a monocyclic or bicycliccycloalkyl, a monocyclic or bicyclic cycloalkenyl, and a monocyclic orbicyclic heterocyclyl. The multicyclic cycloalkenyl is attached to theparent molecular moiety through any carbon atom contained within thebase ring. In certain embodiments, multicyclic cycloalkenyl ringscontain a monocyclic cycloalkenyl ring (base ring) fused to either (i)one ring system selected from the group consisting of a bicyclic aryl, abicyclic heteroaryl, a bicyclic cycloalkyl, a bicyclic cycloalkenyl, anda bicyclic heterocyclyl; or (ii) two rings systems independentlyselected from the group consisting of a phenyl, a monocyclic heteroaryl,a monocyclic cycloalkyl, a monocyclic cycloalkenyl, and a monocyclicheterocyclyl.

As used herein, “cycloalkenylene” refers to a linking cycloalkenylgroup.

As used herein, the term “heteroaryl” refers to a monocyclic, bicyclic,or a multicyclic heteroaryl ring system. The monocyclic heteroaryl canbe a 5 or 6 membered ring. The 5 membered ring consists of two doublebonds and one, two, three or four nitrogen atoms and optionally oneoxygen or sulfur atom. The 6 membered ring consists of three doublebonds and one, two, three or four nitrogen atoms. The 5 or 6 memberedheteroaryl is connected to the parent molecular moiety through anycarbon atom or any nitrogen atom contained within the heteroaryl.Representative examples of monocyclic heteroaryl include, but are notlimited to, furyl, imidazolyl, isoxazolyl, isothiazolyl, oxadiazolyl,oxazolyl, pyridinyl, pyridazinyl, pyrimidinyl, pyrazinyl, pyrazolyl,pyrrolyl, tetrazolyl, thiadiazolyl, thiazolyl, thienyl, triazolyl, andtriazinyl. The bicyclic heteroaryl consists of a monocyclic heteroarylfused to a phenyl, a monocyclic cycloalkyl, a monocyclic cycloalkenyl, amonocyclic heterocyclyl, or a monocyclic heteroaryl. The fusedcycloalkyl or heterocyclyl portion of the bicyclic heteroaryl group isoptionally substituted with one or two groups which are independentlyoxo or thia. When the bicyclic heteroaryl contains a fused cycloalkyl,cycloalkenyl, or heterocyclyl ring, then the bicyclic heteroaryl groupis connected to the parent molecular moiety through any carbon ornitrogen atom contained within the monocyclic heteroaryl portion of thebicyclic ring system. When the bicyclic heteroaryl is a monocyclicheteroaryl fused to a phenyl ring or a monocyclic heteroaryl, then thebicyclic heteroaryl group is connected to the parent molecular moietythrough any carbon atom or nitrogen atom within the bicyclic ringsystem. Representative examples of bicyclic heteroaryl include, but arenot limited to, benzimidazolyl, benzofuranyl, benzothienyl,benzoxadiazolyl, benzoxathiadiazolyl, benzothiazolyl, cinnolinyl,5,6-dihydroquinolin-2-yl, 5,6-dihydroisoquinolin-1-yl, furopyridinyl,indazolyl, indolyl, isoquinolinyl, naphthyridinyl, quinolinyl, purinyl,5,6,7,8-tetrahydroquinolin-2-yl, 5,6,7,8-tetrahydroquinolin-3-yl,5,6,7,8-tetrahydroquinolin-4-yl, 5,6,7,8-tetrahydroisoquinolin-1-yl,thienopyridinyl, 4,5,6,7-tetrahydrobenzo[c][1,2,5]oxadiazolyl, and6,7-dihydrobenzo[c][1,2,5]oxadiazol-4(5H)-onyl. In certain embodiments,the fused bicyclic heteroaryl is a 5 or 6 membered monocyclic heteroarylring fused to either a phenyl ring, a 5 or 6 membered monocycliccycloalkyl, a 5 or 6 membered monocyclic cycloalkenyl, a 5 or 6 memberedmonocyclic heterocyclyl, or a 5 or 6 membered monocyclic heteroaryl,wherein the fused cycloalkyl, cycloalkenyl, and heterocyclyl groups areoptionally substituted with one or two groups which are independentlyoxo or thia. The multicyclic heteroaryl group is a monocyclic heteroarylring (base ring) fused to either (i) one ring system selected from thegroup consisting of a bicyclic aryl, a bicyclic heteroaryl, a bicyclicheterocyclyl, a bicyclic cycloalkenyl, and a bicyclic cycloalkyl; or(ii) two ring systems selected from the group consisting of a phenyl, abicyclic aryl, a monocyclic or bicyclic heteroaryl, a monocyclic orbicyclic heterocyclyl, a monocyclic or bicyclic cycloalkenyl, and amonocyclic or bicyclic cycloalkyl. The multicyclic heteroaryl group isconnected to the parent molecular moiety through any carbon atom ornitrogen atom contained within the base ring. In certain embodiments,multicyclic heteroaryl groups are a monocyclic heteroaryl ring (basering) fused to either (i) one ring system selected from the groupconsisting of a bicyclic aryl, a bicyclic heteroaryl, a bicyclicheterocyclyl, a bicyclic cycloalkenyl, and a bicyclic cycloalkyl; or(ii) two ring systems selected from the group consisting of a phenyl, amonocyclic heteroaryl, a monocyclic heterocyclyl, a monocycliccycloalkenyl, and a monocyclic cycloalkyl. Examples of multicyclicheteroaryls include, but are not limited to5H-[1,2,4]triazino[5,6-b]indol-5-yl,2,3,4,9-tetrahydro-1H-carbazol-9-yl, 9H-pyrido[3,4-b]indol-9-yl,9H-carbazol-9-yl, and acridin-9-yl.

As used herein, “heteroarylene” refers to a linking heteroaryl group.

As used herein, the term “halo” or “halogen” includes fluoro, chloro,bromo, and iodo.

As used herein, “alkoxy” refers to an —O-alkyl group. Example alkoxygroups include methoxy, ethoxy, propoxy (e.g., n-propoxy andisopropoxy), t-butoxy, and the like.

As used herein, “aryloxy” refers to an —O-aryl group. Example aryloxygroups include phenyl-O—, substituted phenyl-O—, and the like.

As used herein, “haloalkyl” refers to an alkyl group having one or morehalogen substituents. Example haloalkyl groups include CF₃, C₂F₅, CHF₂,CCl₃, CHCl₂, C₂Cl₅, and the like.

As used herein, “haloalkenyl” refers to an alkenyl group having one ormore halogen substituents.

As used herein, “haloalkynyl” refers to an alkynyl group having one ormore halogen substituents.

As used herein, “haloalkoxy” refers to an —O-(haloalkyl) group.

As used herein, “heteroalkyl” refers to an alkyl group having at leastone heteroatom such as sulfur, oxygen, or nitrogen.

As used herein, “heteroalkylene” refers to a linking heteroalkyl group.

As used herein, “amino” refers to NH₂.

As used herein, “alkylamino” refers to an amino group substituted by analkyl group.

As used herein, “dialkylamino” refers to an amino group substituted bytwo alkyl groups.

As used herein, “ether” refers to a group comprising an oxygen atomconnected to two alkyl or aryl groups. As used herein, a “vinyl ether”refers to an ether comprising a carbon-carbon double bond bound to theoxygen atom.

As used herein, an “initiator” is a compound capable of initiatingpolymerization or other bond formation. In certain embodiments, theinitiator can form a radical cation. In some embodiments, the initiatorincludes a vinyl ether moiety. In some embodiments, the initiator formsa radical cation in the presence of a photoredox mediator and light.

As used herein, an “organic initiator” refers to a polymerizationinitiator having one or more carbon atoms covalently linked to hydrogen,oxygen, and/or nitrogen. Certain carbon-containing compounds are notconsidered organic: carbides, carbonates and cyanides. As used herein,the organic initiator does not contain metals (i.e., metal-free).

As used herein, an “organic unsaturated initiator” refers to apolymerization initiator having carbon-carbon double bonds orcarbon-carbon triple bonds.

As used herein, a “strained cyclic unsaturated monomer” refers to acyclic or heterocyclic monomer having a ring strain, where the angles ina molecule are compressed or expanded compared to their optimal value.As an example, in bicyclic molecules, the amount of strain energy can bethe sum of the strain energy in each individual ring.

As used herein, an “electron donating substituent” refers to asubstituent that adds electron density to an adjacent pi-system, makingthe pi-system more nucleophilic. In some embodiments, an electrondonating substituent has lone pair electrons on the atom adjacent topi-system. In some embodiments, electron donating substituents havepi-electrons, which can donate electron density to the adjacentpi-system via hyperconjugation. Examples of electron donatingsubstituents include O—, NR₂, NH₂, OH, OR, NHC(O)R, OC(O)R, aryl, andvinyl substituents.

As used herein, a “4-membered carbocyclic intermediate” refers to apolymerization intermediate molecule having a cyclobutane moiety.

As used herein, “electronic conjugation” refers to the overlap of onep-orbital with another across an intervening sigma bond. In transitionmetals, d-orbitals can be involved. A conjugated system has a region ofoverlapping p-orbitals, bridging the interjacent single bonds.Delocalization of pi electrons across all the adjacent alignedp-orbitals can occur, where the pi electrons do not belong to a singlebond or atom, but to a group of atoms.

As used herein, “unsaturated bond” refers to a carbon-carbon double bondor a carbon-carbon triple bond.

As used herein, a “sacrificial co-initiator” refers to a molecule thatoxidizes an organic polymerization initiator. The co-initiator isreduced in the process and is rendered inactive.

As used herein, a “non-sacrificial co-initiator” refers to a mediator.

As used herein, a “mediator” refers to a catalyst that accelerates achemical reaction via an electron transfer. The mediator can participatemultiple times in the electron transfer reaction (oxidation andreduction).

As used herein, a “photoredox mediator” or “photoredox catalyst” is acatalyst that harnesses the energy of light (e.g., visible light) toaccelerate a chemical reaction via an electron transfer. In certainembodiments of the present application, photoredox mediators are organicmolecules, such as pyrylium and acridinium salts.

As used herein, a “monomer” is a substance, each of the molecules ofwhich can, on polymerization, contribute one or more constitutionalunits in the structure of a macromolecule or polymer.

As used herein, the term “copolymer” refers to a polymer that is theresult of polymerization of two or more different monomers. The numberand the nature of each constitutional unit can be separately controlledin a copolymer. The constitutional units can be disposed in a purelyrandom, an alternating random, a regular alternating, a regular block,or a random block configuration unless expressly stated to be otherwise.A purely random configuration can, for example, be:x-x-y-z-x-y-y-z-y-z-zz . . . or y-z-x-y-z-y-z-x-x . . . . An alternatingrandom configuration can be: x-y-x-z-y-x-yz-y-x-z . . . , and a regularalternating configuration can be: x-y-z-x-y-z-x-y-z . . . . A regularblock configuration has the following general configuration: . . .x-x-x-y-y-y-zz-z-x-x-x . . . , while a random block configuration hasthe general configuration: . . . x-x-x-z-z-x-x-y-y-y-y-z-z-z-x-x-z-z-z-. . . .

As used herein, the term “constitutional unit” of a polymer refers to anatom or group of atoms in a polymer, comprising a part of the chaintogether with its pendant atoms or groups of atoms, if any. Theconstitutional unit can refer to a repeat unit. The constitutional unitcan also refer to an end group on a polymer chain. For example, theconstitutional unit of polyethylene glycol can be —CH₂CH₂O—corresponding to a repeat unit, or —CH₂CH₂OH corresponding to an endgroup.

As used herein, the term “repeat unit” corresponds to the smallestconstitutional unit, the repetition of which constitutes a regularmacromolecule (or oligomer molecule or block).

As used herein, the term “end group” refers to a constitutional unitwith only one attachment to a polymer chain, located at the end of apolymer. For example, the end group can be derived from a monomer unitat the end of the polymer, once the monomer unit has been polymerized.As another example, the end group can be a part of a chain transferagent or initiating agent that was used to synthesize the polymer.

As used herein, the term “terminus” of a polymer refers to aconstitutional unit of the polymer that is positioned at the end of apolymer backbone.

As used herein, “living polymerization” refers to a method ofsynthesizing polymers using the well-known concept of additionpolymerization, that is, polymerization wherein monomers are addedone-by-one to an active site on the growing polymer chain but onewherein the active sites for continuing addition of another monomer arenever fully eliminated other than on purpose. That is, the polymer chainis virtually always capable of further extension by the addition of moremonomer to the reaction mixture unless the polymer has been capped,which may be reversible so as permit polymerization to continue orquenched, which is usually permanent. While numerous genera of livingpolymerizations are known, currently the predominant types are anionic,cationic, and radical living polymerizations.

As used herein, a “crosslink” or “cross-linking moiety”, which can beused interchangeably, is a constitutional unit connecting two parts of amacromolecule or polymer.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art. Although methods and materials similar or equivalent to thosedescribed herein can be used in the practice or testing of the presentdisclosure, suitable methods and materials are described below. Allpublications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Incase of conflict, the present specification, including definitions, willcontrol. In addition, the materials, methods, and examples areillustrative only and not intended to be limiting.

Polymerization Methods

Referring to Scheme 3, the ROMP process of the present disclosure isinitiated by one-electron oxidation of an organic initiator, which canbe, for example, a vinyl ether initiator A, to produce an activatedradical cation B. Reaction of the activated radical cation B with astrained cyclic unsaturated monomer forms a [2+2] complex, such as C.Rather than undergoing reductive quenching to generate cyclobutanes,rapid ring-opening to alleviate ring-strain occurs, which completes theROMP initiation event to arrive at D. Continued propagation withadditional strained cyclic unsaturated monomers ultimately yields ROMPpolymers, which can bear a reactive radical cation chain end E.Reductive quenching then provides neutral polymer F. The ROMP processdoes not rely on metal-based catalysts. In some embodiments, the ROMPprocess is free of metals.

Accordingly, in some embodiments, the present disclosure is directed tomethods of making a polymer, including exposing a reaction mixtureincluding a strained cyclic unsaturated monomer and an organic initiatorto a stimulus to provide an activated organic initiator, whereby theactivated organic initiator is effective to polymerize the strainedcyclic unsaturated monomer (e.g., via a 4-membered carbocyclicintermediate) to provide a polymer having constitutional units derivedfrom the strained cyclic unsaturated monomer. The 4-membered carbocyclicintermediate can be formed by a [2+2] cycloaddition of the activatedorganic initiator and the strained cyclic unsaturated monomer.

In another aspect, this disclosure features a method of making a polymerincluding exposing a reaction mixture including a strained cyclicunsaturated monomer, an organic unsaturated initiator, and aco-initiator, to a stimulus to provide an activated co-initiator whichactivates the organic unsaturated initiator, whereby the activatedorganic unsaturated initiator is effective to polymerize the strainedcyclic unsaturated monomer, to provide a polymer having constitutionalunits derived from the strained cyclic unsaturated monomer.

In some embodiments, the reaction mixture can include one or moremonomers including a cycloalkene moiety; an initiator including a vinylether moiety; and optionally a photoredox mediator capable offacilitating electron transfer between the initiator and the monomer;and exposing the mixture to a stimulus (e.g., light, heat, an electricvoltage), to polymerize the monomer.

In some embodiments, the present disclosure provides a method of makinga cross-linked polymer including providing a reaction mixture thatincludes one or more monomers containing a cycloalkene moiety; aninitiator comprising a vinyl ether moiety; and optionally a photoredoxmediator capable of facilitating electron transfer between the initiatorand the monomer; exposing the mixture to light to polymerize the monomerto provide a polymer; and introducing a crosslinking moiety to themixture to crosslink the polymer. In some embodiments, rather thancrosslinking the polymer post-polymerization, the one or more monomerscontaining a cycloalkene moiety can be a crosslinker (e.g., amultifunctional monomer), such that a crosslinked polymer is producedduring polymerization.

Reaction Mixture Components

As discussed above, the reaction mixture that is exposed to a stimulusto provide a polymer or a crosslinked polymer can include a variety ofcomponents, such as organic initiators, co-initiators, mediators,monomers (and co-monomers), and/or crosslinkers. Each of these will beexpanded in detail below.

Organic Initiators

In some embodiments, the organic initiator is an organic unsaturatedinitiator. The organic unsaturated initiator can include, for example,one or more electron-donating substituents in electronic conjugationwith an unsaturated bond.

In some embodiments, the electron-donating substituent is C₁₋₂₀ alkoxy,aryloxy, C₁₋₂₀ alkyl-NH—, aryl-NH—, C₁₋₂₀ alkyl-S—, and/or aryl-S—.

In some embodiments, the electron-donating substituent is C₁₋₁₀ alkoxyl,aryloxy, C₁₋₁₀ alkyl-NH—, aryl-NH—, C₁₋₁₀ alkyl-S—, and/or aryl-S—.

In some embodiments, the electron-donating substituent is C₁₋₆ alkoxyl,aryloxy, C₁₋₆ alkyl-NH—, aryl-NH—, C₁₋₆ alkyl-S—, and/or aryl-S—.

In some embodiments, the organic unsaturated initiator is a compound ofFormula (I)

wherein

R₁ is selected from hydrogen, C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, cycloalkyl,aryl, and heteroaryl, wherein said C₁-C₂₀ alkyl is optionallysubstituted with aryl; and

R₂ is selected from C₁-C₂₀ alkyl, cycloalkyl, aryl, and heteroaryl.

In some embodiments, R₁ is selected from hydrogen, C₁-C₂₀ alkyl, C₂-C₂₀alkenyl, aryl, and heteroaryl; and

R₂ is selected from C₁-C₂₀ alkyl, cycloalkyl, aryl, and heteroaryl.

In some embodiments, R₁ is selected from hydrogen, C₁-C₂₀ alkyl, C₂-C₂₀alkenyl, phenyl, and heteroaryl; and

R₂ is selected from C₁-C₂₀ alkyl, cycloalkyl, aryl, and heteroaryl.

In some embodiments, R₁ is selected from hydrogen, C₁-C₁₀ alkyl, C₂-C₁₀alkenyl, C₃-C₁₀ cycloalkyl, aryl, and heteroaryl, wherein said C₁-C₁₀alkyl is optionally substituted with aryl.

In some embodiments, R₁ is selected from hydrogen, C₁-C₆ alkyl, C₂-C₆alkenyl, C₃-C₆ cycloalkyl, aryl, and heteroaryl, wherein said C₁-C₆alkyl is optionally substituted with aryl.

In some embodiments, R₁ is selected from hydrogen, C₁-C₆ alkyl, C₃-C₆cycloalkyl, aryl, and heteroaryl, wherein said C₁-C₆ alkyl is optionallysubstituted with aryl.

In some embodiments, R₁ is selected from hydrogen, C₁-C₆ alkyl, C₃-C₆cycloalkyl, and aryl, wherein said C₁-C₆ alkyl is optionally substitutedwith aryl.

In some embodiments, R₁ is selected from hydrogen, C₁-C₆ alkyl, C₃-C₆cycloalkyl, and phenyl, wherein said C₁-C₆ alkyl is optionallysubstituted with aryl.

In some embodiments, R₂ is selected from C₁-C₂₀ alkyl, C₃-C₁₀cycloalkyl, aryl, and heteroaryl.

In some embodiments, R₂ is selected from C₁-C₂₀ alkyl, C₃-C₁₀cycloalkyl, and aryl.

In some embodiments, R₂ is selected from C₁-C₁₀ alkyl, C₃-C₁₀cycloalkyl, and aryl.

In some embodiments, R₂ is selected from C₁₋₆ alkyl, C₃-C₆ cycloalkyl,and aryl.

In some embodiments, R₂ is selected from C₁₋₆ alkyl and C₃₋₆ cycloalkyl.

In some embodiments, R₂ is C₁₋₆ alkyl.

It is understood that any of the above embodiments for the definitionsof R₁ and R₂ can be combined to provide a compound of Formula (I).

For example, in some embodiments, R₁ is selected from hydrogen, C₁-C₆alkyl, C₂-C₆ alkenyl, C₃-C₆ cycloalkyl, aryl, and heteroaryl, whereinsaid C₁-C₆ alkyl is optionally substituted with aryl; and R₂ is selectedfrom C₁-C₁₀ alkyl, C₃-C₁₀ cycloalkyl, and aryl.

In some embodiments, R₁ is selected from C₁-C₆ alkyl, C₃-C₆ cycloalkyl,and aryl, wherein said C₁-C₆ alkyl is optionally substituted with aryl;and R₂ is selected from C₁-C₁₀ alkyl, C₃-C₁₀ cycloalkyl, and aryl.

In some embodiments, R₁ is selected from C₁-C₆ alkyl, C₃-C₆ cycloalkyl,and aryl, wherein said C₁-C₆ alkyl is optionally substituted with aryl;and R₂ is selected from C₁-C₁₀ alkyl and C₃-C₁₀ cycloalkyl.

In some embodiments, R₁ is selected from C₁-C₆ alkyl, C₃-C₆ cycloalkyl,and aryl, wherein said C₁-C₆ alkyl is optionally substituted with aryl;and R₂ is C₁-C₁₀ alkyl.

In some embodiments, the organic unsaturated initiator is selected from

The organic initiator can be activated by oxidation. For example, theactivated organic initiator (i.e., oxidized organic initiator) caninclude a cationic radical.

In some embodiments, instead or in addition to an organic unsaturatedinitiator, the organic initiator includes an organic photoinitiator. Forexample, the organic photoinitiator can be selected from

The organic photoinitiator is capable of bond formation when exposed tolight.

The organic initiator to monomer (and co-monomer) ratio in a reactionmixture can range from 1:20 (e.g., from 1:50, from 1:100, from 1:200,from 1:500, from 1:1000, from 1:2000, from 1:3000, or from 1:4000) to1:5000 (e.g., to 1:4000, to 1:3000, to 1:2000, to 1:1000, to 1:500, to1:200, to 1:100, or to 1:50). For example, the organic initiator tomonomer (and co-monomer) ratio can range from 1:20 to 1:1000, from 1:100to 1:1000, from 1:200 to 1:2000, from 1:1000 to 1:5000, or from 1:1000to 1:3000. In some embodiments, the organic initiator to monomer (andco-monomer) ratio is 1:20. In certain embodiments, the organic initiatorto monomer ratio is 1:5000. In certain embodiments, the organicinitiator to monomer (and co-monomer) ratio is 1:100, 1:200, 1:500,1:1000, 1:2000, 1:3000, or 1:4000.

Co-Initiators (Mediators)

In some embodiments, as discussed above, the present disclosure featuresa method of making a polymer including exposing a reaction mixturecomprising a strained cyclic unsaturated monomer, an organic unsaturatedinitiator, and a co-initiator, to a stimulus to provide an activatedco-initiator which activates the organic unsaturated initiator toprovide a polymer having constitutional units derived from the strainedcyclic unsaturated monomer. The co-initiator can be sacrificial ornon-sacrificial. The sacrificial co-initiator refers to a molecule thatoxidizes an organic polymerization initiator, and is reduced in theprocess and is rendered inactive. The non-sacrificial co-initiator canalso be a mediator.

In some embodiments, a non-sacrificial co-initiator (i.e., a mediator, aphotoredox mediator) is a pyrylium salt, an acridinium salt, athiopyrylium salt, a 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, and/or apersulfate salt.

In some embodiments, the non-sacrificial co-initiator can be a compoundof Formula (II), (III), or (IV):

wherein:

R₃ is each independently selected from H, C₁₋₆ alkyl, C₁₋₆ alkoxy, andaryl; and

X⁻ is a counterion.

wherein:

R₄ is each independently selected from H, C₁₋₆ alkyl, C₁₋₆ alkoxy, andaryl; and

X⁻ is a counterion.

wherein:

R₉ is C₁₋₁₀ alkyl; and

X⁻ is a counterion.

In some embodiments, R₃ is each independently selected from H, C₁₋₃alkyl, C₁₋₃ alkoxy, and aryl.

In some embodiments, R₃ is each independently selected from H, CH₃,OCH₃, and phenyl.

In some embodiments, R₄ is each independently selected from H, C₁₋₃alkyl, C₁₋₃ alkoxy, and aryl.

In some embodiments, R₄ is each independently selected from H, CH₃,OCH₃, and phenyl.

In some embodiments, R₉ is C₁₋₆ alkyl.

In some embodiments, R₉ is methyl.

In some embodiments, X⁻ is BF₄ ⁻ or ClO₄ ⁻.

In some embodiments, X⁻ is BF₄ ⁻.

In some embodiments, R₃ is each independently selected from H, CH₃,OCH₃, and phenyl; and

X⁻ is BF₄ ⁻.

In some embodiments, R₄ is each independently selected from H, CH₃,OCH₃, and phenyl; and

X⁻ is BF₄ ⁻.

In some embodiments, the compound of Formula (IV) is

In certain embodiments, the non-sacrificial co-initiator (i.e., themediator, the photoredox mediator) is a compound of Formula (V)

wherein R₅ is each selected from alkyl, alkenyl, alkynyl, and aryl, andX⁻ is a counterion.

In some embodiments, R₅ is each selected from alkyl, alkenyl, andalkynyl.

In some embodiments, R₅ is each selected from branched or straight-chainalkyl groups having greater than six carbons.

In some embodiments, R₅ is each selected from C₁₋₁₀ alkyl, C₂₋₁₀alkenyl, and C₂₋₁₀ alkynyl.

In some embodiments, R₅ is each selected from C₁₋₁₀ alkyl and C₂₋₁₀alkenyl.

In some embodiments, R₅ is each selected from C₁₋₁₀ alkyl.

In some embodiments, R₅ is each independently selected from C₁₋₆ alkyl.

In some embodiments, R₅ is each independently selected from C₁₋₆ alkyland phenyl.

In some embodiments, X⁻ is BF₄ ⁻ and ClO₄ ⁻.

In some embodiments, R₅ is each selected from alkyl, alkenyl, andalkynyl.

In some embodiments, R₅ is each selected from branched or straight-chainalkyl groups having greater than six carbons; and X⁻ is BF₄ ⁻ and ClO₄⁻.

In some embodiments, R₅ is each selected from C₁₋₁₀ alkyl, C₂₋₁₀alkenyl, and C₂₋₁₀ alkynyl; and X⁻ is BF₄ ⁻ and ClO₄ ⁻.

In some embodiments, R₅ is each selected from C₁₋₁₀ alkyl and C₂₋₁₀alkenyl; and X⁻ is BF₄ ⁻ and ClO₄ ⁻.

In some embodiments, R₅ is each selected from C₁₋₁₀ alkyl; and X⁻ is BF₄⁻ and ClO₄ ⁻.

In some embodiments, R₅ is each independently selected from C₁₋₆ alkyl;and X⁻ is BF₄ ⁻ and ClO₄ ⁻.

In some embodiments, R₅ is each independently selected from C₁₋₆ alkyland phenyl; and X⁻ is BF₄ ⁻ and ClO₄ ⁻.

In some embodiments, the non-sacrificial co-initiator is a compound ofFormula (VI)

wherein:

R₆ is each independently selected from C₁₋₂₀ alkyl, C₂₋₂₀ alkenyl,phenyl, aryl, and heteroaryl groups, and

Y⁻ is a counterion.

In some embodiments, R₆ is each independently selected from aryl orheteroaryl optionally substituted with 1, 2, or 3 substituents eachindependently selected from alkyl, alkoxy, —O-alkenyl, and —O-alkynyl.

In some embodiments, R₆ is each independently aryl or heteroaryloptionally substituted with 1, 2, or 3 substituents each independentlyselected from C₁₋₆ alkyl, C₁₋₆ alkoxy, —O—C₂₋₆ alkenyl, and —O—C₂₋₆alkynyl.

In some embodiments, R₆ is aryl optionally substituted with 1, 2, or 3substituents each independently selected from C₁₋₆ alkyl, C₁₋₆ alkoxy,—O—C₂₋₆ alkenyl, and —O—C₂₋₆ alkynyl groups.

In some embodiments, R₆ is aryl optionally substituted with 1, 2, or 3substituents each independently selected from C₁₋₆ alkyl, C₁₋₆ alkoxy,and —O—C₂₋₆ alkenyl.

In some embodiments, R₆ is aryl optionally substituted with 1, 2, or 3substituents each independently selected from C₁₋₆ alkyl and C₁₋₆alkoxy.

In some embodiments, R₆ is aryl optionally substituted with 1, 2, or 3substituents each independently selected from C₁₋₃ alkyl and C₁₋₃alkoxy.

In some embodiments, Y is an anion selected from BF₄ ⁻ and ClO₄ ⁻.

The co-initiator to monomer (and co-monomer) ratio in a reaction mixturecan range from 1:100 (e.g., from 1:500, from 1:1000, from 1:5000, from1:10,000, from 1:50,000, from 1:100,000, from 1:500,000, or from1:750,000) to 1:1,000,000 (e.g., to 1:750,000, to 1:500,000, to1:100,000, to 1:50,000, to 1:10,000, to 1:5,000, to 1:1000, or to1:500). For example, the organic initiator to monomer (and co-monomer)ratio can range from 1:100 to 1:10,000, from 1:100 to 1:100,000, from1:100 to 1:1,000,000, from 1:1000 to 1:500,000, from 1:10,000 to1:100,000, or from 1:10,000 to 1:500,000. In some embodiments, theco-initiator to monomer (and co-monomer) ratio is 1:100. In certainembodiments, the organic initiator to monomer ratio is 1:1,000,000. Incertain embodiments, the organic initiator to monomer (and co-monomer)ratio is 1:100, 1:500, 1:1000, 1:5000, 1:10,000, 1:50,000, 1:100,000,1:500,000, or 1:750,000.

In some embodiments, instead of a non-sacrificial co-initiator, thereaction mixture includes a sacrificial co-initiator, which can be anoxidizing agent. For example, the sacrificial co-initiator can beNa₂SO₅, KHSO₅, Na₂S₂O₈, and/or (NH₄)₂S₂O₈. The sacrificial co-initiatorcan oxidize the initiator so that the initiator can react with amonomer.

In some embodiments, the sacrificial or non-sacrificial co-initiator issoluble in an organic solvent and/or miscible with the monomers of thepresent disclosure. When the sacrificial or non-sacrificial co-initiatoris soluble or miscible in monomers, and the mixture of sacrificial ornon-sacrificial co-initiator and monomers is liquid at either ambient orelevated temperatures, then the polymerization reaction can occur in theabsence of a solvent.

When the polymerization reaction is solvent-free, the overall efficiencyof material production can be increased (by eliminating solvent costs,and solvent removal procedures after the polymerization), and thepolymerization reaction can allow for materials such as cross-linkedphoto-cured resins to be produced without void spaces or impurities thatcan be caused by solvent entrapment.

Monomers (and Co-Monomers)

As discussed above, the reaction mixture includes a strained cyclicunsaturated monomer. In some embodiments, the strained cyclicunsaturated monomer has a ring strain of at least 20 kcal/mol. In someembodiments, the strained cyclic unsaturated monomer is a strainedcycloalkene. The strained cycloalkene can be, for example, norbornene,cyclobutene, cyclooctene, cyclodecene, cyclododecatriene, and/orderivatives thereof.

In some embodiments, the strained cycloalkene is

In some embodiments, the strained cycloalkene is

In some embodiments, the strained cycloalkene is branched. In someembodiments, the strained cycloalkene includes a bicyclic [2.2.1]heptane moiety.

In some embodiments, polymers with pendant alcohol and silyl groups canbe made using the methods of the present disclosure, by providing areaction mixture having a monomer including a cycloalkene moiety thatincludes a silyl ether.

In certain embodiments, the monomer that includes a cycloalkene moietyhaving a silyl ether has a Formula (VII)

wherein R₇ is selected from alkyl, alkenyl, alkynyl, and poly(ethyleneoxide), and R₈ is independently selected from hydrogen, alkyl, alkenyl,and alkynyl. The silyl ether-containing monomer can be used as the solemonomer in a reaction mixture, or in combination with other monomers, inany ratio.

In some embodiments, R₇ is selected from C₁₋₆ alkyl, C₂₋₆ alkenyl, C₂₋₆alkynyl, and polyethylene oxide.

In some embodiments, R₇ is selected from C₁₋₆ alkyl, C₂₋₆ alkenyl, andpolyethylene oxide.

In some embodiments, R₇ is selected from C₁₋₆ alkyl and polyethyleneoxide.

In some embodiments, R₈ is independently selected from hydrogen, C₁₋₆alkyl, C₂₋₆ alkenyl, and C₂₋₆ alkynyl, provided that at least one R₈ isnot hydrogen.

In some embodiments, R₈ is independently selected from hydrogen, C₁₋₆alkyl, and C₂₋₆ alkenyl, provided that at least one R₈ is not hydrogen.

In some embodiments, R₈ is independently selected from hydrogen and C₁₋₆alkyl provided that at least one R₈ is not hydrogen.

In certain embodiments, the silyl monomer has the formula

The silyl ether-containing monomers can be readily polymerized either asthe sole monomers of the reaction or as part of a mixture of monomersusing the methods of the present disclosure, and can provide alkyl-,dialkyl-, or trialkylsiloxy oligomers or polymers.

Furthermore, the silyl ether group can be readily removed using standardprotocols such that the silyl ethers in the final polymer can bepartially or entirely converted into a hydroxyl functional group (toproduce a polyalcohol). An illustrative example for polymerization of asilyl ether-containing monomer is provided in Example 4, below.

Crosslinkers

The methods of the present disclosure can provide a crosslinked polymerthat is based upon hydrocarbons and/or that is substantially free ofmetals.

For example, after polymerization, the methods can further includecrosslinking the polymer. The crosslinkers have two or more reactivegroups, such as thiol, hydroxy, amino, or carboxylic acid groups. Insome embodiments, the crosslinkers further include a spacer in betweenthe two or more reactive groups, such as an arylene, an alkylene, adiarylsulfanyl, or a polyethylene glycol spacer.

In some embodiments, crosslinking the polymer includes reacting thepolymer with a crosslinker selected from:

In some embodiments, rather than crosslinking a polymer in apost-polymerization reaction, a crosslinked polymer can be made duringpolymerization, using multifunctional monomers. For example, a reactionbased upon hydrocarbons and/or that substantially excludes metals canhave a reaction mixture that further includes monomers having at leasttwo cycloalkene moieties, which results in a direct, single stepcrosslinking to produce a crosslinked polymer. The crosslinked polymercan have high toughness and durability, and a one-step synthesis ofcrosslinked polymer can be amenable to photo-curing processes and vatphoto-polymerizations. In certain embodiments, the reaction mixtures ofthe present disclosure include a strained cyclic unsaturated monomerhaving a single unsaturated moiety and a multifunctional monomer havingtwo or more cycloalkene moieties. The ratio between a strained cyclicunsaturated monomer having a single unsaturated moiety and amultifunctional monomer can be from about 10,000:1 to about 10:1 (e.g.,from about 10,000:1 to about 10:1, from about 10,000:1 to about 100:1,from about 10,000:1 to about 1,000:1, from about 1,000:1 to about 10:1,from about 1,000:1 to about 100:1, or from about 100:1 to about 10:1).

In some embodiments, the multifunctional monomers include at least twocycloalkene moieties coupled through a linker. In certain embodiments,such a monomer has the formula (VIII):

wherein L is a linker selected from alkylene, alkenylene, alkynylene,polysiloxane, and poly(ethylene oxide).

In some embodiments, L is a linker selected from C₁₋₂₀ alkylene, C₂₋₂₀alkenylene, C₂₋₂₀ alkynylene, polysiloxane, and poly(ethylene oxide).

In some embodiments, the multifunctional monomer is

Stimulus

In some embodiments, the stimulus used to initiate the ROMP reaction islight, heat, and/or an electric potential. The light can be, forexample, ultraviolet light having a wavelength of about 250-350 nmand/or visible light having a wavelength of about 350-750 nm. The heatcan be, for example, a temperature of up to about 150° C. (e.g., fromabout 25° C. to about 150° C., from about 40° C. to about 150° C., fromabout 60° C. to about 150° C., or from about 80° C. to about 150° C.).The electric potential can be, for example, from 0 to +2.5 V (e.g., 0 to+2 V, 0.5 to +2.5 V) vs. saturated calomel electrode (SCE).

In certain embodiments, a source of the light is a blue LED. In certainother embodiments, the light source is a white light. The white lightcan include fluorescent bulbs and digital light processing (DLP)projectors, including commercially available DLP projectors. DLPprojectors can enable polymerization processes in which the DLPprojector projects specific images that dictate the shape of thepolymerized material. Methods using light sources can further includelayer-by-layer protocols or additive manufacturing (three-dimensionalprinting) by vat polymerization. The additive manufacturing process caninclude polymerizing a reaction mixture of the present disclosure byexposing the reaction mixture to light in a layer-by-layer manner toprovide a three-dimensional object.

For example, in some embodiments, the additive manufacturing processincludes providing a mixture including a cycloalkene moiety-containingmonomer, an initiator having a vinyl ether moiety, and a photoredoxmediator capable of facilitating electron transfer between the initiatorand the monomer; exposing a first portion of the mixture to light topolymerize a first portion of the mixture; and exposing a second portionof the mixture adjacent to the first polymerized portion of the mixtureto light to polymerize a second portion of the mixture. Thepolymerization is continued in a layer-by-layer manner to provide athree-dimensional object.

Polymerization Conditions

In certain embodiments, the methods of the present disclosure areconducted under oxygen-free and/or water-free environments. Theoxygen-free and/or water-free environment can be accomplished with aninert-atmosphere dry-box, filled with, for example, nitrogen or argongas, and/or with anhydrous solvent and other anhydrous reagents.

In certain other embodiments, the methods of the present disclosure areconducted in ambient conditions, or other oxygen-containing and/orwater-containing conditions. In some embodiments, the reaction mixturecan contain about 1% or less of water. For example, when reagent gradesolvent is used without any protocols to remove water, and thepolymerization is conducted in an open container in ambient atmosphere,the polymerization can achieve conversion of monomer and final polymermolecular weight similar to those obtained under air-free conditions.The ability to effect polymerization under ambient conditions isimportant as it greatly simplifies the technical aspects of thepolymerization, gives important insights into the reactivity of thereagents, and enables a broader scope of applications. Specificapplications may include systems for reaction injection molding,additive manufacturing (3D printing), or other applications in whichon-demand curing is desirable.

Polymer Properties

The polymers made by the methods of the present disclosure can have analkenyl substituted with a C₁-C₂₀ alkoxy moiety at a polymer terminus.The polymers can be colorless, white, or beige in coloration. In someembodiments, the polymer is metal-free.

Compared to metal-catalyzed ROMP polymers, the polymers made by themethods of the present disclosure have increased stability (i.e., areless susceptible to degradation), are not as darkly colored, and can befunctionalized with a variety of functional groups, such as ester andalcohol moieties.

Articles

The polymers made by the methods of the present disclosure can be usedto make a variety of articles. For example, the polymers can beincorporated into dental implants, vehicle components (vehicle bodyparts), corrosion-resistant casings, protective eye equipment, ballisticimpact resistant materials (e.g., ballistic panels), prosthetics,orthotics, athletic equipment, electronic devices, and/or opticsdevices.

Cross Metathesis and Ring-Closing Metathesis

While the methods of the present disclosure are useful in ROMPreactions, they can also be used for performing other reactions. Scheme4 provides generalized depictions of the reactions that can be achievedusing the methods of the present disclosure for olefin metathesis.

In each of the reactions shown in Schemes 4A to 4D, an organic initiatoror catalyst, such as a vinyl ether, is used. Oxidation of the organicinitiator or catalyst leads to formation of a [2+2] complex with analkene reactant and subsequent breakdown of the complex results informal olefin metathesis. In some embodiments, R_(A), R_(B), R_(C), andR_(D) groups can each independently be alkyl or aryl groups, which maybe optionally substituted. In some embodiments (e.g., Schemes 4A and4B), a photoredox catalyst is used in the reaction.

Scheme 4A shows an olefin cross metathesis (CM) using a stoichiometric(1:1) molar ratio of a vinyl ether and alkene. Scheme 4B shows ametathesis reaction in which the vinyl ether and alkene moieties aretethered, such that the intramolecular reaction accomplishes aring-closing metathesis (RCM) event. In some embodiments, both the CMand RCM reactions can be accomplished using catalytic amounts of vinylether, as depicted in Scheme 1C and D. Although the method is notlimited to vinyl ethers bearing only an ethenyl group, the use of vinylethers benefits from formation of gaseous ethylene (CH₂CH₂) which helpsto drive the reactions toward completion. Without wishing to be bound bytheory, it is believed that the ability of the vinyl ether to exchange Rgroups via metathesis events facilitates catalytic turnover.

The following examples are provided to illustrate, not limit, theinvention.

Example 1 provides an electro-organic ROMP method for making polymers.Example 2 represents a protocol for organic-initiated ROMP. The approachutilizes one-electron oxidation of electron-rich vinyl ethers toinitiate the process, which can be achieved either electrochemically orvia photoredox processes. As will be described below, a photoredoxapproach enabled high yields of polymerization in short reaction timesunder mild conditions. The methods of the present application enableunique synthetic control over end group functionality. The success ofthe photoredox mediation provides new opportunities for spatiotemporalcontrol over production of ROMP-based polymers and materials. Example 3demonstrates the preparation of linear, non-crosslinkedpolydicyclopentadiene using a photoredox-mediated organic-initiated ROMPprocedure. The monomer, endo-DCPD, can also be copolymerized withnorbornene to prepare polymers with varied amounts of cyclopenteneunits. Example 4 represents a protocol for polymerizing a silylether-containing monomer and for deprotecting the resulting polymer.

EXAMPLES Example 1. Electro-Organic ROMP

Electro-organic ROMP (eo-ROMP) is believed to undergo the mechanisticsteps described previously in Scheme 3, where an e-electron anodicoxidation of a vinyl ether A produces the activated radical cation B;subsequent formation of a [2+2] complex C, followed by fragmentation andring-opening to alleviate ring strain completes the ROMP initiationevent arriving at D. Continued propagation with additional cycloalkanemonomers ultimately yields ROMP polymers, which can bear a reactiveradical cation chain end E. Reductive quenching provides the neutralspecies F.

Referring to Scheme 5, eo-ROMP was demonstrated with a series ofstrained monomers 1a-1e and vinyl ether initiators 2a-2c. Cyclicvoltammograms (CVs) of the initiators showed oxidation potentials(E_(ox)) between 1.43 and 1.30 V (vs SCE). Polymerization conditionsincluded using an undivided cell with a carbon fiber anode and cathode,non-aqueous reference electrode (Ag/AgNO₃), CH₃NO₂ as solvent(tetrahydrofuran, dioxane, CH₂Cl₂ and other organic co-solvents may alsobe used), and LiClO₄ (1.0 M) as supporting electrolyte. Polymerizationswere conducted using a constant potential typically between 1.4 and 1.8V (vs SCE). Polymerizations were conducted under nitrogen atmosphereusing anhydrous reagents and solvents. The initial monomer concentration([M]₀) was 1.5 M and initial initiator concentration ([I]₀) was 0.015 M.

TABLE 1 Polymers from eo-ROMP. Monomer Initiator [M]₀/[I]₀ M_(w) (kDa)1a 2a 100/1  25.2 1b 2b 83/1 14.1 1c 2c 93/1 12.2 1d 2a 100/1  8.6 1e 2c96/1 4.2

Homogeneous electrochemical mediators were able to facilitate theeo-ROMP method at lower cell potentials. Specifically, the use oftriarylamines such as triphenylamine, tris(4-bromophenyl)amine, andtris(4-nitrophenyl)amine could be used to effect eo-ROMP at oxidationpotentials as low as 1.0 V vs SCE. The use of the mediator was carriedout by using a 2:1 molar ratio of alkene initiator to mediator. Themediator was added at the beginning of the polymerization, and the cellpotential was then held constant during the polymerization.

Representative Example of Eo-ROMP without Electrochemical Mediators

All reactions were done under a nitrogen atmosphere. In a 3-neck roundbottom flask, 1.60 g (15.0 mmol) LiClO₄, 2.12 g (22.5 mmol) norbornene,and 0.03 mL (0.270 mmol) ethyl propenyl ether were dissolved in 15 mL ofCH₃NO₂ (norbornene was not fully soluble). The flask was capped withsepta containing a carbon fiber working electrode, double junctionreference electrode (0.1 M TBAB/0.01 M AgNO₃), and a carbon fibercounter electrode. A constant potential of 1.30 V vs SCE was appliedwith stirring of the solution. Once the reaction was complete, 0.200 gof hydroquinone was added to the solution. The solution was stirred for10 minutes before being poured into MeOH with vigorous stirring. Theprecipitate was collected via vacuum filtration, washed with MeOH, anddried under vacuum.

Representative Example of Eo-ROMP with an Electrochemical Mediator

All reactions were done under a nitrogen atmosphere. In a 3-neck roundbottom flask, 1.60 g (15.0 mmol) LiClO₄, 0.065 g (0.135 mmol)tris(4-bromophenyl) amine, 2.12 g (22.5 mmol) norbornene, and 0.03 mL(0.270 mmol) ethyl propenyl ether were dissolved in 15 mL of CH₃NO₂(norbornene was not fully soluble). The flask was capped with septacontaining a carbon fiber working electrode, double junction referenceelectrode (0.1 M TBAB/0.01 M AgNO₃), and a carbon fiber counterelectrode. A constant potential of 1.01 V vs SCE was applied withstirring of the solution. The solution turned blue immediately uponelectrolysis. Once the reaction was complete, 0.200 g of hydroquinonewas added to the solution. The solution was stirred for 10 minutesbefore being poured into MeOH with vigorous stirring. The precipitatewas collected via vacuum filtration, washed with MeOH, and dried undervacuum.

Example 2. Photoredox ROMP

Norbornene (1) was used as a monomer, as this scaffold exhibitsrelatively high ring strain among common ROMP monomers (Scheme 6). Toinvestigate the direct oxidation of vinyl ethers and propensity for theensuing radical cation to initiate ROMP, bulk electrolysis was conductedon solutions of 1 containing vinyl ether initiators 2a-2c. Afterelectrolysis of 1 and 2a for 3 h, the reaction solution and electrodeswere analyzed for any presence of polynorbornene (PNB). A small amountof material was obtained (3% yield), and analysis by NMR spectroscopyrevealed signals consistent with PNB. Moreover, end group signalsconsistent with the vinyl ether were observed even after precipitationof the polymer to remove any residual small molecule initiator. Gelpermeation chromatography (GPC) analysis revealed a number-averagemolecular weight (M_(n)) of 11.8 kDa (Ð=2.2). Similar results wereobtained using initiators 2b and 2c. Although the yields of PNB werelow, the results confirm that an anodic oxidation of 2 could initiatepolymerization of 1 to give polymer structures consistent with a ROMPmechanism.

Having confirmed the overall reactivity and viability of an electrolyticROMP protocol, a photoredox polymerization was carried out.

Photoredox initiation and control of polymerizations can be a powerfulmethod for achieving spatiotemporal control over polymerizations andorganic-initiated methods. Notably, photoredox polymerization strategieshave focused almost exclusively on controlled radical additionpolymerizations in which a redox process is inherent in theactivation/deactivation of the polymer chain end. Traditionalmetal-mediated ROMP, on the other hand, is redox-neutral at all stages,and the metal complex is covalently attached to each chain end untilchemically cleaved at the end of the polymerization.

Pyrylium and acridinium salts 3a-3c (Scheme 7) have been identified asgood candidates for facilitating photo-oxidation. These mediators arecapable of facilitating electron transfer when in the photo-excitedstate, and were expected to be good oxidizers for the vinyl etherinitiators 2a-2c. Whereas the initiators 2a-c display oxidationpotentials in the range of 1.43 to 1.30 V vs SCE, the oxidizing power ofexcited state pyrylium and acridinium cations have been calculated to be1.74 and 2.06 V vs SCE, respectively. To explore the photoredoxinitiation of ROMP, an initial monomer concentration of ca. 2.3 M inCH₂Cl₂ was used with a monomer to initiator (1:2a) molar ratio of ca.100:1. Using a blue LED light source (λ=450-480 nm) in the presence of3, the best yields were obtained from the pyrylium tetrafluoroboratesalt 3a. In general, 3b gave lower yields than 3a, and 3c did notproduce any detectable PNB. Thus, all additional experiments wereconducted using 3a. The structure of the PNB was confirmed by ¹H NMRanalysis in comparison with an authentic sample prepared via traditionalROMP using the Grubbs 1^(st)-generation initiator. The glass transitiontemperature (T_(g)) of samples prepared by either traditional orphotoredox mediated ROMP were also found to be consistent with oneanother. Specifically, the T_(g) of the ROMP polymer prepared byRu-mediated ROMP (M_(n)=49.5 kDa) was found to be 53.3° C., versus 49.5°C. for a sample prepared by photoredox initiation (M_(n)=43.9 kDa).

Each initiator 2a-2c was found to give PNB in good yield via thephotoredox method; as shown in Table 2. In absence of blue LED light,but with exposure to ambient lighting from the fume hood, slowconversion to give PNB was observed (Table 2, entry 11). In completeabsence of light, no polymer was observed. In general, polymerizationsunder optimized conditions were found to reach maximum conversion in ca.30-150 min.

TABLE 2 Polymerization results and GPC data for photoredox mediated ROMP

conver- initi- [M]₀ sion time M_(n′ theo) M_(n,exp) entry ator 1:2:3^(a)(M)^(b) (%)^(c) (min) [kDa] [kDa] Ð  1 2a  97:1:0.03 2.3 88 (73)  30 8.015.1 1.7  2 2b  97:1:0.03 2.3 92 (80)  30 8.4 14.9 1.6  3 2c  106:1:0.032.4 87 (67)  30 9.0 15.8 1.6  4 2c  104:1:0.10 2.3 80 (76) 150 7.8 13.51.4  5 2c  104:1:0.25 2.3 90 (73) 120 8.9 19.2 1.6  6 2c  48:1:0.03 2.295 (78)  60 4.3  8.1 1.4  7 2c  57:1:0.03 1.3 93 (58) 120 5.0 11.5 1.4 8 2c  491:1:0.03 5.3 51 (25) 120 23.6  22.2 1.5  9 2c  494:1:0.03 2.272 (50)  60 33.4  43.9 1.5 10 2c 1000:1:0.03 2.3 61 (47) 120 57.4  60.21.6 11^(d) 2c  103:1:0.03 2.3 53 (29) 2580  5.0  7.2 1.3 ^(a)Initialmolar ratios of monomer, initiator, and mediator. ^(b)Initial monomerconcentration. ^(c)Conversion of monomer, as determined by 1H NMRanalysis; isolated yields after precipitation given in parentheses.^(d)Reaction mixture exposed to ambient light from fume hood, withoutexposure to blue LED light source. M_(n,theo) is theoreticalnumber-average molecular weight calculated from initial monomer toinitiator ratio and % conversion of monomer. M_(n,exp) is experimentalnumber-average molecular weight, calculated from a weight-averagemolecular weight determined by GPC using multi-angle laser lightscatters. Dispersities (D) determined by GPC analysis.

The amount of photoredox mediator 3a that was required for successfulpolymerization was found to be quite low. Specifically, consistent M_(n)values and % conversions were observed when using mediator to initiatorratios (3a:2) of 0.03 to 0.25 (Table 2, entries 3-5). Higher loading ofmediator did manifest some bimodality in the GPC traces, with highmolecular weight shoulders appearing with increasing amounts ofmediator. It is believed that this may be due to increased concentrationof active chain ends and therefore greater extent of chain-chaincoupling. The initial monomer concentration could be varied, with evenvery high concentrations (5.3 M) giving successful polymerizations(entry 8). This indicates that bulk polymerization using liquid monomersis possible using the methods of the present invention. Varying theinitial monomer to initiator ratio provided some degree of control overthe final M_(n) (entries 3, 6-10). A consistent correlation was observedbetween the theoretical and experimental M_(n) values, with experimentalvalues generally being greater than expected for the given monomer toinitiator ratios and % conversions. Dispersities were found to varybetween 1.3 and 1.7 across different experiments, and remainedconsistent during the course of each polymerization.

During the course of the polymerization, a gradual increase in M_(n) wasobserved with increasing conversion of monomer, consistent with thechain growth nature of ROMP (FIGS. 1A and 1B). Although there was apositive correlation, the linearity was not as precise as traditional“living” ROMP using, for example, Grubbs 3^(rd)-generation initiator.This could be ascribed to the relative rates of initiation andpropagation in the photoredox method, or any number of early terminationevents.

Mechanistically, without wishing to be bound by theory, it is believedthat oxidation of the vinyl ether proceeds via electron transfer to theexcited pyrylium cation to give the vinyl ether radical cation (e.g.,A→B, Scheme 3, above). Notably, the propagating radical cation chain endlikely forms a dynamic redox couple with the reduced pyrylium species.The reversibility would manifest an ability to reductively quench andterminate polymerization, and then reinitiate upon exposure to bluelight. This temporal control was investigated by monitoring thepolymerization with intermittent exposure to blue LED light. As shown inFIG. 2, polymerization ceased in the dark and was reinitiated uponexposure to blue light. Specifically, little to no further conversion ofmonomer in the dark was observed as determined by ¹H NMR spectroscopy,and no significant changes in M_(n) was observed as judged by GPCanalysis. This suggested that the pyrylium cation and vinyl ether form adynamic redox couple and that the radical cation chain end isreductively quenched during the polymerization. Furthermore, thecorrelation between % conversion and increasing M_(n) during thealternating light/dark cycles was consistent with chain endactivation/deactivation cycles, as opposed to photo-mediated initiationof new polymer chains upon re-exposure to light.

The organic-initiated ROMP approach utilizes one-electron oxidation ofelectron-rich vinyl ethers to initiate the process, which can beachieved either electrochemically or via photoredox processes. Aphotoredox approach enabled high yields of polymerization in shortreaction times under mild conditions.

Synthetic Procedure

Acetonitrile (CH₃CN) and nitromethane (CH₃NO₂) were dried over calciumhydride and distilled prior to use. Dichloromethane (CH₂Cl₂) andtetrahydrofuran (THF) were obtained from a solvent purification system.¹H and ¹³C NMR spectra were recorded on Bruker AVance 300 MHz or 500 MHzspectrometers. Chemical shifts are reported in delta (δ) units,expressed in parts per million (ppm) downfield from tetramethylsilaneusing the residual protio-solvent as an internal standard (CDCl₃, ¹H:7.27 ppm and ¹³C: 77.0 ppm). Data are presented as follows: chemicalshift, multiplicity (s=singlet, d=doublet, dd=doublet of doublets,br=broad, m=multiplet), coupling constants (Hz) and integration.UV-visible spectroscopy data were collected on an Agilent 8453 UV-visspectrophotometer. Gel permeation chromatography (GPC) was performedusing a GPC setup consisting of: a Shimadzu pump, 3 in-line columns, andWyatt light scattering and refractive index detectors withtetrahydrofuran (THF) as the mobile phase. Number-average molecularweights (M_(n)) and weight-average molecular weights (M_(w)) werecalculated from light scattering. All polymerizations were carried outunder an inert atmosphere of nitrogen in standard borosilicate glassvials purchased from Fisher Scientific with magnetic stirring unlessotherwise noted. Irradiation of photochemical reactions was done using a2 W Miracle blue LED indoor gardening bulb purchased from Amazon.Electrochemical experiments were performed on a CH Instruments 1100Bpotentiostat using a 25 mL 3-neck round bottom flask as an undividedcell. Cyclic voltammetry experiments were done using a glassy carbonworking electrode (3 mm diameter), Pt counter electrode (Premier LabSupply), and Ag/0.01 M AgNO₃ (0.1 M tetrabutylammonium tetrafluoroboratein CH₃CN) reference electrode. Electro-organic ROMP experiments weredone using a carbon fiber (Zoltek) working electrode, carbon fibercounter electrode, and Ag/0.01 M AgNO₃ (0.1 M tetrabutylammoniumtetrafluoroborate in CH₃CN) reference electrode. T_(g) values weredetermined using a Perkin-Elmer DMA 8000. Analysis was performed onpowdered samples held within material pockets supplied by Perkin-Elmer.Samples were analyzed using the Single-Cantilever Geometry Fixture withthe following settings: heating rate=3.0° C./min, frequency=1 Hz, staticforce=1.0 N. Reported T_(g) values refer to the temperaturecorresponding to the peak of the tan delta curve. Initiators 2a and 2bwere prepared according to literature procedures. The pyryliumtetrafluoroborate (3a) and perchlorate (3b) salts were preparedaccording to literature procedures. All other reagents and solvents wereobtained from commercial sources and used as received unless otherwisenoted.

General Procedure for the Preparation of Initiators 2a and 2b

A solution of potassium tert-butoxide (5.1 g, 45.0 mmol, 1.5 equiv.) in10 mL of dry THF was slowly added to a solution of(methoxymethyl)triphenylphosphonium chloride (15.4 g, 45.0 mmol, 1.5equiv.) in 40 mL of dry THF. After stirring the red solution at 23° C.for 45 min, a solution of the corresponding aldehyde (30.0 mmol, 1.0equiv.) in 10 mL of dry THF was slowly added and allowed to stir at 23°C. for an additional 2 h. The solvent was removed under vacuum and theresidue was diluted with hexanes. The organic layer was washed withwater (3×100 mL) and dried over Na₂SO₄. The solvent was removed underreduced pressure and the resulting residue was purified by filteringthrough a plug of silica with diethyl ether as the eluent. In somecases, residual triphenylphosphine was removed by stirring overnightwith 10 equiv of iodomethane and filtration through a plug of silicawith diethyl ether as the eluent.

1-methoxy-4-phenyl Butene (2a)

was prepared according to the above procedure in 92% yield (1:2 cis totrans ratio); spectral data were consistent with literature values.

2-cyclohexyl-1-methoxyethylene (2b)

was prepared according to literature procedures in 82% yield (1:2 cis totrans ratio). ¹H NMR (300 MHz, CDCl₃) δ=6.29 (d, J=12 Hz, 1 H, trans)5.79 (d, J=6.0 Hz, 0.5 H, cis) 4.70 (dd, J=6.0 Hz, 9 Hz, 1 H, trans)4.24 (dd, J=3 Hz, 6 Hz, 0.5 H, cis) 3.58 (s, 1.5 H, cis) 3.50 (s, 3 H,trans) 2.42 (m, 0.5 H, cis) 1.88 (m, 1 H, trans) 1.68 (m, 4 H,cis/trans) 1.16 (m, 9 H, cis/trans). ¹³C NMR (125 MHz, CDCl₃) δ=145.7,144.5, 113.4, 109.7, 59.5, 55.8, 36.9, 34.4, 35.4, 33.4, 26.2, 26.1,26.0.

To a flask containing p-anisaldehyde (6.1 mL, 50.3 mmol, 1 equiv) andp-acetylanisole (15.07 g, 100.4 mmol, 2 equiv) was added BF₃·Et₂O (15.0mL, 121.5 mmol, 2.4 equiv) dropwise over 5 min. The solution was heatedin an oil bath set to 100° C. After 2 h, the reaction was removed fromheat. Once at room temperature, the crude material was diluted withacetone (200 mL) and Et₂O (250 mL) and filtered to give a rust-coloredsolid. The solids were washed with warm acetone (175 mL) and dried undervacuum to give the pyrylium tetrafluoroborate as an orange solid (5.01g, 20%). Spectral data matched those previously reported.

2,4,6-tri-(p-methoxyphenyl) pyrylium perchlorate (3b)

was prepared according to literature procedures in 10% yield, spectraldata were consistent with literature values.

General Procedure for Cyclic Voltammetry of Initiators 2a-2c

The general procedure was as follows: In a drybox, a 3-neck round bottomflask was charged with a magnetic stir bar, anhydrous CH₃NO₂ (15 mL),and lithium perchlorate (15.0 mmol). The indicated initiator (0.075mmol) was then added to the mixture. The flask was equipped with aglassy carbon anode (3 mm diameter), Pt basket cathode, and Ag/AgNO₃reference electrode (0.01 M AgNO₃/0.1 M tetrabutylammoniumtetrafluoroborate in CH₃CN) and then the apparatus was sealed usingrubber septa. The electrochemical cell was then removed from the dryboxand the solution was placed under a positive pressure of N₂ and stirredat room temperature. Stirring was stopped prior to connecting to thepotentiostat. The cyclic voltammograms for the initiators were typicallytaken from 0.5 V to 2.5 V vs. Ag/AgNO₃ with a sweep rate of 0.10 V/s.Ferrocene (0.15 mmol) was added as an internal standard after eachvoltammogram. All potentials are reported in V vs. SCE.

General procedure for electro-organic ROMP. Electro-organic ROMPexperiments were done using a carbon fiber (Zoltek) working electrode,carbon fiber counter electrode, and Ag/0.01 M AgNO₃ (0.1 Mtetrabutylammonium tetrafluoroborate in CH₃CN) reference electrode in adouble junction chamber. The carbon fiber electrodes were 40 mm inlength (excluding the copper lead) and 15 mm of the carbon fiber wassubmerged in the electrolyte solution during electrolysis. In thedrybox, a 3-neck round bottom flask was charged with a magnetic stirbar, lithium perchlorate (15.0 mmol) and CH₃NO₂ (15 mL). To the solutionwas added norbornene (23.2 mmol, 100 equiv.) and initiator (0.23 mmol, 1equiv.). The electrodes were attached onto the cell and the apparatuswas sealed using rubber septa. The electrochemical cell was then removedfrom the dry box and placed under a positive pressure of N₂. Theelectrodes were connected to the potentiostat and bulk electrolysis witha constant potential of 1.43 V vs SCE was started with constantstirring. After the current reached background levels, the electrolysiswas stopped and hydroquinone (2.3 mmol, 10 equiv) was then added to thesolution. The carbon fiber electrodes were removed and soaked in THF.The quenched solution and electrode soaks were added to rapidly stirringmethanol to precipitate the polymer. The resulting solids wereredissolved in THF, passed through a syringe filter (2 μm) to remove anycarbon fiber particulates, and reprecipitated into methanol. Theresulting solids were dried under vacuum and analyzed by ¹H NMRspectroscopy and GPC.

General procedure for photoredox mediated ROMP. All polymerizations wereset up in a drybox under an inert atmosphere of nitrogen. Irradiation ofthe sealed vials with blue LEDs was done outside of the drybox. A 2-dramvial was equipped with a magnetic stir-bar, 2,4,6-tri-(p-methoxyphenyl)pyrylium tetrafluoroborate (3a, 3.0-25.0 mol %), and norbornene (48-1000equiv. relative to 2). The solvent, CH₂Cl₂ (2.2-2.5 M), and initiator 2(1 equiv.) were added to the vial. The vial was sealed with aTeflon-coated screw cap and brought out of the drybox. The mixture wasirradiated for the indicated period of time. The reaction progress andM_(n) were monitored by ¹H NMR spectroscopy and GPC, respectively. Uponcompletion, hydroquinone (5 equiv.) was added to the reaction mixture,which was then passed through a short plug of alumina. The polymersolution was then added dropwise into an excess of methanol (MeOH) ordry acetonitrile (CH₃CN) to cause precipitation of the polymer. Note: Asa control, the same setup was performed outside the drybox, and then thereaction mixture sparged with N₂ for 15 minutes before irradiation. Thisled to a significant decrease in polymer formation (only ˜30% conversionof monomer as determined by ¹H NMR spectroscopy) compared to reactionssetup inside the drybox.

Example 3. Photoredox ROMP with Co-Monomers

Polymers with more complex functionality led to examination of a varietyof other common ROMP monomers. Specifically, the polymerization ofdicyclopentadiene (DCPD, 2, Scheme 10) was examined, which in itscommercially available form exists as almost exclusively the endoisomer. While catalyst systems can enable the preparation of linearpolyDCPD (2→3, Scheme 10, bottom), many catalysts form insoluble,crosslinked polymer networks resulting from either olefin metathesis orolefin addition reactions of the cyclopentene moiety (Scheme 10, top).Furthermore, the nature of the crosslinking process means that the metalcatalysts used for polymerization remain trapped inside the finalpolymer, which can be problematic in some cases.

Monomer 2 was found to be successfully polymerized using enol etherinitiator 4 and photoredox mediator 5 upon exposure to blue light in anorganic-initiated ROMP (Scheme 2, bottom). Under these conditions,conversion of 2 was found to be only 15%, and the polymer that wasformed was of low molecular weight (Mn=3.8 kDa; Ð=1.1). In comparison,the use of norbornene (1) as monomer often leads to conversions of >80%.The polyDCPD (3) remained soluble in common organic solvents (e.g., THF,CH₂Cl₂, and toluene) and showed no signs of crosslinking by 1H-NMRanalysis.

Attempts to optimize this polymerization to achieve higher conversionare outlined in Table 3. Notably, independently varying the initialmonomer concentration (entries 1-3) or pyrylium (5) loading (entry 4)resulted in no significant changes in conversion. Carrying out thepolymerization at 4° C. gave a slight improvement, as did decreasing theinitial ratio of monomer to initiator. In all cases, the molecularweights of the final polymer remained low.

TABLE 3 Polymerization of dicyclopentadiene. entry 2:4:5^([a]) [2]₀(M)^([b]) temp. (° C.) Conversion^([c]) 1 102:1:0.07 1.75 23 15% 2100:1:0.07 2.80 23 13% 3 101:1:0.07 1.26 23 13% 4 102:1:0.25 1.75 23 15%5 100:1:0.07 1.75 4 19% 6  51:1:0.07 1.76 23 20% ^([a])Initial molarratio of 2, 4, and 5. ^([b])Initial concentration of 2 in CH₂Cl₂.^([c])Conversion determined by comparison of monomer and polymer peaksby ¹H-NMR spectroscopy.

Initially, the presence of monomer 2 was evaluated to determine whetherits presence was detrimental to the polymerization of norbornene (1).Copolymers derived from monomers 1 and 2 were prepared using a feedratio of monomers (i.e., 1+2) to initiator 4 of 100:1. FIG. 3 shows goodcorrelation of endo-DCPD (2) loading on the composition and Mn of thefinal polymer. Although the amount of 2 incorporated is less than thetheoretical amount based upon the feed ratio, the % incorporation showsa consistent increase with increasing endo-DCPD content. As expected,higher initial loadings of monomer 2 led to a significant decrease inthe Mn of the final polymer from 18.8 kDa (˜10% DCPD) to 4.1 kDa (˜90%DCPD) which were accompanied by significantly lower conversions andisolated yields. Nevertheless, this highlights how organic-initiatedROMP can be amenable to the tuning of materials properties (e.g., Tg)through the preparation of copolymers. Interestingly, in contrast to thelow conversion of 2 at high loadings of this monomer, when small amountsof 2 were present, this monomer displayed conversions of 50-60%suggesting that decreased reactivity of the monomer was not the causefor the low conversions observed with higher DCPD loadings.

In an attempt to better understand the reasons for the low conversion,two potential explanations were considered. The first scenario involvesthe bulk of the extra cyclopentene ring in monomer 2 (compared withnorbornene 1), which can deter polymerization from proceeding to highconversion through a steric effect. The radical cation likely approachesthe monomer's convex face opposite this cyclopentene ring, making stericinteractions in the monomer unlikely for decreased conversion. However,the endo orientation results in a ring-opened structure where thepropagating chain end is syn to the cyclopentene ring, which mayattenuate the rate of new monomer incorporation (FIG. 4, top).Alternatively, the presence of the second olefin in the monomer maycreate problems due to its proximity to the propagating radical cation(FIG. 4, middle). The intramolecular reactivity of neighboring olefinswith radical cation intermediates is well-documented, and even utilizedfor the development of cascade type reactivity. Notably, these undesiredside reactions could arise either during formation of the cyclobutaneradical cation (C, Scheme 3, above), or through the subsequent ringopened intermediate (E, Scheme 3, above) during propagation.

To probe these two possible pathways, monomers 6-8 were prepared inorder to compare their performance with 2 (FIG. 4, bottom). Theexo-configuration of monomer 6 would be expected to perform well if theproblem was strictly sterics, whereas monomers 7 and 8 were chosen toremove the possibility of any undesired intramolecular reactivity withthe extra olefin. Previous studies on the polymerization of monomers 2,6, and 7 using Ru-alkylidene ROMP catalysts have found that exo-DCPD (6)polymerizes approximately 20 times faster than endo-DCPD (2). Thus,while this effect is primarily steric in nature, coordination of the Rucatalyst by the cyclopentyl olefin does occur to a small extent. Incontrast, it is believed that the poor behavior of 2 underorganic-initiated conditions was most likely due to undesired reactivityof the proposed radical intermediates given the known proclivity ofthese species to undergo intramolecular reactions with olefins.

Monomers 2, 6, 7, and 8 each undergo polymerization to varying degrees(FIG. 5). The exo-DCPD monomer (6) was found to perform poorly (<20%conversion), analogously to what was observed with the endo-isomer. Incontrast, the endo-dihydroDCPD monomer (7) performed significantlybetter, typically reaching 50-60% conversion. Unfortunately, theresulting polymer appeared to display poor solubility indichloromethane, which likely contributes to conversion not proceedingpast this point. Finally, exo-dihydroDCPD (8) performed exceptionallywell, reaching >90% conversion. This level of conversion is on par withwhat was previously seen with the parent norbornene monomer 1. Takentogether, the success of monomers 7 and 8 as well as the poorperformance of monomers 2 and 6 suggested that the low conversions ofthe latter monomers can be ascribed to the extra unsaturation in thecyclopentene moiety and not steric impedance.

Based on control experiments, it does not appear that thephotoredox-mediated ROMP is intolerant of all olefinic groups. Whennorbornene (1) was polymerized using enol ether 4 in the presence ofcyclopentene (ratio of cyclopentene:1:4=25:75:1), a conversion of 79%was observed for norbornene, consistent with examples where cyclopenteneis absent. Additionally, no incorporation of cyclopentene was observedby 1H-NMR analysis of the final polymer. This provides further evidencethat the conversion-limiting process in the polymerizations of endo- andexo-DCPD is an intramolecular process. In addition to demonstrating newmonomers that can be utilized for organic-initiated ROMP, these studiesalso provide insight into mechanistic considerations with regards tofuture monomer design.

The ability to prepare linear polyDCPD that has not undergonecrosslinking is beneficial in terms of the processability of thematerial and the ability to control when crosslinking occurs. Currenttechnologies typically utilize Reaction Injection Molding (RIM), wherethe monomer and initiator are injected directly into a mold andpolymerize to form a molded, crosslinked polymer. The possibility ofisolating the linear polymer and then carrying out a subsequent reactionto form crosslinked polyDCPD under fully organic-initiated conditionswas explored (FIG. 6). Thiol-ene reactivity was used to achieve thecrosslinking due to the mild conditions, high reactivities, and tunableproduct properties that have been demonstrated with this approach.Irradiation of a THF solution of polymer 3 in the presence of dithiol 9and photoinitiator 10 with a hand held UV lamp (λ=365 nm,) led togelation within 30 minutes (FIG. 6).

Materials and Methods

Dichloromethane (CH₂Cl₂) and tetrahydrofuran (THF) were obtained from asolvent purification system. ¹H and ¹³C NMR spectra were recorded onBruker AVance 300 MHz or 500 MHz spectrometers. Chemical shifts arereported in delta (δ) units, expressed in parts per million (ppm)downfield from tetramethylsilane using the residual protio-solvent as aninternal standard (CDCl₃, ¹H: 7.26 ppm and ¹³C: 77.0 ppm). Data arereported as follows: chemical shift, multiplicity (s=singlet, d=doublet,dd=doublet of doublets, br=broad, m=multiplet), coupling constants (Hz)and integration. Gel permeation chromatography (GPC) was performed usinga GPC setup consisting of: a Shimadzu pump, 3 in-line columns, and Wyattlight scattering and refractive index detectors with tetrahydrofuran(THF) as the mobile phase. Number-average molecular weights (M_(n)) andweight-average molecular weights (M_(w)) were calculated from lightscattering. All polymerizations were carried out under an inertatmosphere of nitrogen in standard borosilicate glass vials purchasedfrom Fisher Scientific with magnetic stirring unless otherwise noted.Irradiation of photochemical reactions was done using a 2 W Miracle blueLED indoor gardening bulb purchased from Amazon. T_(g) values weredetermined using a Perkin-Elmer DMA 8000. Analysis was performed onpowdered samples held within material pockets supplied by Perkin-Elmer.Samples were analyzed using the Single-Cantilever Geometry Fixture withthe following settings: heating rate=3.0° C./min, frequency=1 Hz, staticforce=1.0 N. Reported T_(g) values refer to the temperaturecorresponding to the peak of the tan delta curve. The pyryliumtetrafluoroborate (5) salt was prepared according to literatureprocedure. Monomer 2 was dissolved in Et₂O, filtered over neutralalumina, and concentrated prior to use. Tetrahydrofuran (THF) forcrosslinking studies was filtered over neutral alumina and sparged withN₂ for 10 minutes prior to use. All other reagents and solvents wereobtained from commercial sources and used as received unless otherwisenoted.

Preparation of Monomers and Purity

Commercially available monomer 2 was found to contain approximately 2%of exo-isomer 6 by ¹H-NMR analysis. This material was dissolved in Et₂O,filtered over neutral alumina, and concentrated prior to use. Monomer 6was prepared according Nelson, G. L.; Kuo, C.-L. Synthesis 1975, 105-106and found to contain approximately 3% of endo isomer 2 by ¹H-NMRanalysis. Monomer 7 was prepared according to Masjedizadeh, M. R. etal., J. Org. Chem. 1990, 55, 2742-2752 and found to contain <2% ofexo-isomer 8 by ¹H-NMR analysis.

Referring to Scheme 11, monomer 8 was prepared according to PCTpublication WO2009/003711 with modifications. The material obtainedusing this method was found to contain approximately 8% of endo-isomer 7based on ¹H-NMR analysis.

To a 200 mL flask was added endo-dicyclopentadiene (2, 40.73 g, 308mmol, 1 equiv) followed by HBr (48% aqueous, 68 mL, 601 mmol, 2 equiv).The reaction was heated to 70° C. and maintained for 14 hours. Aftercooling to room temperature, the mixture was diluted with water (150mL), and extracted with Et₂O (3×125 mL). The combined organic layerswere washed with saturated aqueous NaHCO₃ (75 mL) and dried over MgSO4.The product was purified by distillation under reduced pressure (52° C.,250 mtorr) to give SI-1 as a pale-yellow oil (50.0 g, 76% yield), whichwas utilized for subsequent transformations.

A flask containing HBr adduct SI-1 (15.03 g, 70.5 mmol, 1 equiv) 10%Pd/C (1.5 g, 1.41 mmol Pd, 0.02 equiv) and EtOAc (30 mL) was evacuatedand backfilled with H₂ (balloon) a total of five times and allowed tostir under H₂. The reaction was periodically analyzed using ¹H NMRspectroscopy until no more olefin signals were present (if conversionceased prior to complete disappearance, additional Pd/C was added). Oncethe olefin signals were gone, the system was evacuated and backfilledwith N₂ a total of 4 times and the liquid filtered over celite (EtOAceluent). The solvent was removed under reduced pressure and the crudematerial taken directly onto the next step.

To a flask containing the crude oil was added KOH (11.9 g, 212.1 mmol, 3equiv) as a solution in 95% EtOH (45 mL). The mixture was heated toreflux. After 21 hours, the reaction was cooled to room temperature,diluted with water (150 mL) and extracted with Et₂O (2×100 mL). Thecombined organic layers were washed with water (3×75 mL) and dried overMgSO₄. The crude product was purified by distillation under reducedpressure (51-53° C., 10 torr) to give 8 as a colorless oil (4.91 g, 52%yield, 2 steps). Spectral data matched those previously reported.

Copolymerizations Utilizing Norbornene and Endo-Dicyclopentadiene

General Procedure: A 2 dram vial containing a magnetic stirbar andp-OMeTPT (5, 1.6 mg, 0.003 mmol, 0.07 equivs) was taken into a gloveboxmaintained under nitrogen atmosphere. To this vial were added norbornene(1) and endo-dicyclopentadiene (2) (1+2=4.5 mmol, 100 equiv).Dichloromethane (2 mL) was added, followed by ethyl propenyl ether (5μL, 0.045 mmol, 1 equiv). The vial was capped, removed from theglovebox, and irradiated with blue LEDs (λ=450-480 nm) for 5 hours. Asmall scoop of hydroquinone was added to the vial and an aliquot takenfor analysis to determine conversion of each monomer. The contents ofthe vial were then diluted with CH₂Cl₂ and filtered over neutral aluminato remove any remaining p-OMeTPT. This CH₂Cl₂ mixture was concentrateddown to approximately 5 mL and precipitated into MeOH (100 mL). Thesolids were collected by filtration, washed with MeOH, and dried underreduced pressure to give the final polymer.

TABLE 4 Conversion and molecular weight data for norbornene andendo-DCPD copolymers. norbornene DCPD % % NB DCPD Total Isolated (NB, 1)(2) DCPD DCPD M_(n) conv. conv. conv. Yield T_(g) Entry (equivs)(equivs) (feed) (polymer) (kDa) Ð (%) (%) (%) (%) (° C.) 1 90 10 10 618.8 1.6 81 60 79 66 54.5 2 80 21 20.8 14 12.9 1.3 72 51 68 43 62.4 3 7330 29.1 20 9.8 1.4 54 35 49 38 66.2 4 62 40 39.2 27 7.6 1.3 36 32 35 3071.7 5 50 50 50 33 7.5 1.3 33 22 29 21 78.4 6 40 60 60 45 6.2 1.2 40 2229 17 84.6 7 30 71 70.3 53 5.5 1.2 37 16 22 16 91.4 8 24 83 77.6 61 5.41.2 36 15 20 11 98.9 9 11 91 89.2 77 4.1 1.2 41 10 13 9 110.8 10 0 100100 100 3.8 1.1 — 14 14 9 118.3Tracking Monomer Conversion vs. Time

General Procedure: A 2 dram vial containing a magnetic stirbar andp-OMeTPT (5, 1.6 mg, 0.003 mmol, 0.07 equivs) was taken into a gloveboxmaintained under nitrogen atmosphere. To this vial were added thedesired monomer (4.5 mmol, 100 equiv). Dichloromethane (2 mL) was added,followed by ethyl propenyl ether (5 μL, 0.045 mmol, 1 equiv). The vialwas capped, removed from the glovebox, and irradiated with blue LEDs(λ=450-480 nm). Aliquots were removed at the designated time points byopening the vial under a heavy cone of N₂ and diluted with CDCl₃saturated with hydroquinone for ¹H-NMR analysis. Time points refer tothe total amount of irradiation time experienced by the sample.

TABLE 5 Conversion vs. time data for monomers 2, 6, 7, and 8. Time(min.)

 1  2%  4%  7% 15%  3  5%  5% 17% 33%  6  7%  8% 26% 53% 10 11%  9% 34%70% 15 14% 12% 41% 78% 20 15% 15% 45% 85% 25 18% 17% 49% 88% 30 19% 20%50% 90% 40 24% 21% 51% 93% 50 25% 22% 53% 96% 75 28% 22% 54% 96%UV-Promoted Thiol-Ene Crosslinking of Polynorbornene Derivatives

General Procedure: To a vial containing the polymer dissolved in THFwhich had been filtered over neutral alumina and then sparged with N₂(100 mg/mL) was added 2,2′-(Ethylenedioxy)diethanethiol (9, 0.25 equivbased on monomer molecular weight) and2,2-dimethoxy-2-phenylacetophenone (0.1 equiv based on monomer molecularweight). The vial was irradiated using a handheld UV lamp (4 W, λ=365nm) without stirring for 30 minutes. The solution was observed to haveformed a gel and no longer flowed when the vial was inverted.

Thus, Example 3 demonstrates the ability to prepare linear,non-crosslinked polydicyclopentadiene using a photoredox-mediatedorganic-initiated ROMP procedure. The monomer, endo-DCPD, can also becopolymerized with norbornene to prepare polymers with varied amounts ofcyclopentene units. The low conversion observed with this monomer wasfound to be due to the presence of the additional olefin moiety, and twopartially hydrogenated monomers were shown to reach high conversionunder the polymerization conditions. Finally, the ability to crosslinkthe polyDCPD was demonstrated in a manner that avoids metal-basedreagents throughout the entire process.

Example 4. Polymerization of Silyl Ether-Containing Monomers andDeprotection of Resulting Polymers

Polymerization

A vial containing 2,4,6-tris(4-methoxyphenyl)pyrylium tetrafluoroborate(1.5 mg, 0.003 mmol) and a stirbar was taken into an inert atmosphereglovebox. Norbornene (324.3 mg, 3.44 mmol) was added, followed bybicyclo[2.2.1]hept-5-en-2-yl)methoxy)(tert-butyl)dimethylsilane (273.0mg, 1.14 mmol), dichloromethane (2 mL), and ethyl propenyl ether (5 μL,0.045 mmol). The vial was sealed and removed from the glovebox. The vialwas irradiated with blue LEDs for 2 hours. The reaction mixture wasfiltered over neutral alumina using dichloromethane. The polymer wasprecipitated into methanol to give the final polymer (260.0 mg, 44%yield) as a white, fluffy solid. NMR analysis showed that the TBS ethercontent was ˜17%.

Deprotection

To a solution of the TBS ether containing polymer (99.5 mg) in THF (8mL) at 0° C. was added tetra-n-butylammonium fluoride (TBAF, 1.0M inTHF, 200 μL). After 3 hours, the reaction was concentrated, and theresidue washed repeatedly with methanol to provide the final polymer(49.5 mg).

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The invention claimed is:
 1. An additive manufacturing process,comprising: exposing a first layer of a reaction mixture comprising astrained cyclic unsaturated monomer and an organic initiator to light toprovide a polymerized first layer of the reaction mixture; exposing thereaction mixture, in a second layer adjacent to the first layer, tolight to provide a polymerized second layer of the reaction mixture; andcontinuing to expose the reaction mixture in subsequent adjacent layersrelative to an immediately preceding layer to polymerize the reactionmixture in a layer-by-layer manner to provide a three-dimensionalobject, wherein exposing the reaction mixture to light provides anactivated organic initiator, whereby the activated organic initiator iseffective to polymerize the strained cyclic unsaturated monomer via a4-membered carbocyclic intermediate, to provide a polymer havingconstitutional units derived from the strained cyclic unsaturatedmonomer.
 2. The method of claim 1, wherein the organic initiator ismetal-free or is an organic unsaturated initiator.
 3. The method ofclaim 2, wherein the organic unsaturated initiator comprises one or moreelectron-donating substituents in electronic conjugation with anunsaturated bond, and the electron-donating substituent is selected fromC₁₋₂₀ alkoxy, aryloxy, C₁₋₂₀ alkyl-NH—, aryl-NH—, C₁₋₂₀ alkyl-S—, andaryl-S—.
 4. The method of claim 2, wherein the organic unsaturatedinitiator is a compound of Formula (I)

wherein R₁ is selected from hydrogen, C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl,aryl, and heteroaryl groups; and R₂ is selected from C₁-C₂₀ alkyl,cycloalkyl, aryl, and heteroaryl groups.
 5. The method of claim 2,wherein the organic unsaturated initiator is selected from


6. The method of claim 1 wherein the organic, initiator is an organicphotoinitiator selected from


7. The method of claim 1, further comprising oxidizing the organicinitiator.
 8. The method of claim 1, wherein the reaction mixturefurther comprises an oxidizing agent, a mediator, or both an oxidizingagent and a mediator.
 9. The method of claim 8, wherein the mediator isselected from pyrylium salts, acridinium salts, thiopyrylium salts, and2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
 10. The method of claim 1,wherein the polymerization is conducted under ambient atmosphere. 11.The method of claim 1, wherein the stimulus is selected from ultravioletlight or visible light.
 12. The method of claim 1, wherein the strainedcyclic unsaturated monomer has a ring strain of at least 20 kcal/mol.13. The method of claim 12 wherein the strained cyclic unsaturatedmonomer is a strained cycloalkene selected from norbornene, cyclobutene,cyclooctene, cyclodecene, and cyclododecatriene.
 14. The method of claim13, wherein the strained cycloalkene is selected from


15. The method of claim 1, further comprising crosslinking the polymer.16. The method of claim 15, wherein crosslinking the polymer comprisesreacting the polymer with a crosslinker selected from


17. An additive manufacturing process, comprising: exposing a firstlayer of a reaction mixture comprising a strained cyclic unsaturatedmonomer, an organic unsaturated initiator, and a co-initiator to lightto provide a polymerized first layer of the reaction mixture; exposingthe reaction mixture, in an adjacent layer to the first layer, to lightto provide a polymerized second layer of the reaction mixture; andcontinuing to expose the reaction mixture in subsequent adjacent layersrelative to an immediately preceding layer to polymerize the reactionmixture in a layer-by-layer manner to provide a three-dimensionalobject, wherein exposing the reaction mixture to light provides anactivated co-initiator which activates the organic unsaturatedinitiator, whereby the activated organic unsaturated initiator iseffective to polymerize the strained cyclic unsaturated monomer via a4-membered carbocyclic intermediate, to provide a polymer havingconstitutional units derived from the strained cyclic unsaturatedmonomer.
 18. The method of claim 17, wherein the co-initiator isselected from pyrylium salts, acridinium salts, thiopyrylium salts,2,3-dichloro-5,6-dicyano-1,4-benzoquinone, persulfate salts.
 19. Themethod of claim 17, wherein the co-initiator is selected from

wherein R₃ and R₄ is each independently selected from H, C₁₋₆ alkyl,C₁₋₆ alkoxy, and aryl.
 20. The method of claim 17, wherein theco-initiator is selected from Na₂SO₅, KHSO₅, Na₂S₂O₈, and (NH₄)₂S₂O₈.21. A three-dimensional object, comprising a metal-free polymercomprising an alkenyl substituted with a C₁-C₂₀ alkoxy moiety at apolymer terminus.
 22. The three-dimensional object of claim 21, whereinthe object is made according to the method of claim
 1. 23. Thethree-dimensional object of claim 21, wherein the object is madeaccording to the method of claim 17.